There doesn't seem to be a direct explanation, but I would say "yes" it is because of the thing you mentioned.

In all of their individual encounters prior to forming their circle, they were almost taken, and in those cases it was pretty quick. King's prose slows down time a little, but they all freeze up or almost lose the ability to act, but then some certain something always gets their feet moving. Usually it is described as coming from outside of themselves.

Stan even starts reciting bird species that acts like a magic spell that actually saves him.

The Loser's Club itself is always described in magical or ritualistic terms, and that they were chosen as a foil. So they have some limited protection of some sort, and there clearly is an explicit "cosmic battle" element that has not been properly delineated.

Once they each got away, and later formed the circle, that's when It starts to try and psyche them out. It may be because now they are a threat and It is dwelling on them. It doesn't otherwise seem to care who it kills--in the Black Spot incident, Mike's dad said It killed one of the Derry Decency guys--so the fixation on them is probably because of their "power."

Still a lot of questions though. It is never really made clear why It would need to give up the chase or not simply come to their house. In other words, there aren't any bright lines such as a need for darkness or privacy even.

As a minor side note: I am in the middle of the book (again) now (kind of coincidentally with the new movie), and I don't seem to recall anyone yet saying anything about It feeding on fear. Then again, I wasn't looking for it.

They are able to exploit It's weaknesses

"It's Weaknesses" from Stephen King wikia:

Despite It seeing Itself as the superior being and stating that Its brain embraces the whole continent, It is far from being almighty. Though It does seem to have significant power over Derry and the town's citizens, It displays several weaknesses which the Losers exploit.

For instance, It clearly underestimates and scorns all of mankind, including the Losers. It is notable in many cases that It leaves an open escape route for the victims and lets them run away. This was seen when the young Ben Hanscom encountered the mummy and when Eddie saw the leper under the porch of 29 Neibolt Street. As a result of this superiority complex, It constantly makes mistakes and does illogical things. When Henry Bowers and his sidekicks chase the Losers into the sewer tunnel system, It attacks Henry's gang instead and turns on the Losers after killing two of Henry's friends. It is also mentioned in the novel that It killed a child named Frederick Cowan by emerging from the toilet, and yet It was unable to finish off the Losers one by one using this same method because It didn't believe that It needed to do so in order to kill them.

It is a psychically sensitive entity, so courage and heart can overcome It, even in its most diabolical forms. Once the Losers are together, their strong shared will and love for each other successfully overpower It and Its fiendish machinations. Their strong faith in their various methods of fighting It eventually lead them to victory. The Losers' assault on 29 Neibolt Street forced It to quickly retreat after being hit by a silver slug (because of the Losers' commonsense solution of using silver against supernatural entities).

The novel also states that when It transforms into a shape, It must obey the laws of that form. This clearly means that It is not invulnerable, and its physical forms can bleed and can be significantly damaged and perhaps even destroyed.

It goes into hibernation for approximately 30 years between killing cycles. During Its hibernation, It may be extremely vulnerable to surprise attacks. However, despite having been defeated for good, it has been heavily implied in other books (such as the Dreamcatcher and Hearts in Atlantis) that It may be still alive. It's natural enemy, The Turtle "Maturin," is mentioned in The Dark Tower series and the character Father Callahan even managed to defend himself from a large group of demons using a cross and the Turtle image, in spite of the latter having "died" during this novel. So, it can be speculated that only a part of Its form was destroyed.

They are guided by Ka to kill IT

It is pretty heavily implied and widely accepted that The Losers' Club are brought together by ka into a ka-tet. This essentially means that they are brought together by destiny for a specific purpose. That purpose is to defeat Pennywise.

"Ka" from Stephen King Wikia:

Ka is the will of Gan, roughly synonymous with destiny or fate.

Ka has been described as "a wheel [whose] only purpose is to turn."

Ka may be considered to be a guide or a destination, though it is not a strict plan. The machinations of ka can, with effort, be obstructed, though more often than not, ka still wins out in the end. Ka is not necessarily a force of good or evil, as it manipulates the powers of both and seems to have no definitive morality of its own.

Further, ka "signifies life-force, consciousness, duty and destiny. In the vulgate, or low speech, it also means a place to which an individual must go."

Individuals drawn together by the will of ka for a purpose are known as ka-tet.

"Ka-tet" from Stephen King Wikia:

Ka-tet is a term in High Speech for a group of people drawn together by ka for a purpose.

The gunslinger Roland Deschain describes ka-tet as being "one from many." Susannah Holmes derives an even simpler definition of the term: it is the notion of family. A tet often consists of several central individuals bound for a purpose, with additional individuals playing transitory roles and specific functions within the group.

Groups that might be considered ka-tets which were never specifically identified as such include:

• It: Bill Denbrough, Mike Hanlon, Ben Hanscom, Richie Tozier, Stan Uris, Eddie Kaspbrak, and Beverly Marsh
• The Stand: Stu Redman, Larry Underwood, Glen Bateman, and Ralph Brentner
• Dreamcatcher: Gary Jones, Henry Devlin, Joe Clarendon, and Pete Moore
• Cell: Clayton Riddell, Tom McCourt, Alice Maxwell, Ray Huizenga, Daniel Hartwick, and Denise Link
• Under the Dome: Dale Barbara, Julia Shumway, Rose Twitchell, Rusty Everett, and Joe McClatchey

They are protected by the Other (Gan)

While the ideas of Ka and Ka-tet indicate that the group was brought together with the purpose of killing It, as noted in the above definitions, this doesn't necessarily mean there was a plan, and this alone isn't necessarily what gave them power over It.

It is alluded to in the novel that they are given protection by the Other also known as Gan who is essentially the supreme being or God of the Stephen King multiverse.

It is continually surprised by the Losers' victories over it and near the end, It begins to question if It is not as superior as it had once thought. However, It doesn't believe that the individual children are strong enough to defeat it. Only with "the Other" working through them as a group does It believe they can harm it.

Actually IT tries to kill them all in the neibolt street house but it all goes wrong and IT was hurt very badly . That comes as a surprise to IT, and first time in his lifetime IT discover a new feeling "A Fear".(Test of his own medicine :P)

Fear to be hurt again,Fear to be killed.

That is why IT use new strategy after that incident. He influenced henry and asked him to kill all 7 of them for him.

UPDATE

All characters were attacked by IT but they all survived these attacks in book.

I will give you some brief from the book (I read It 3 years ago so If I mistaken please feel free to update)

FROM BOOK :

1.Ben Hanscom (Attacked by Mummy) - He had been attacked by IT in Mummy form near the bridge. IT lures him to come near to the bridge but at the last moment Ben realize that some thing is wrong and he just run fast as he can to his home.

2.Richie Tozier and Bill Dunbrough (Attacked by wolfman) - They both went to the neibolt street house where they were attacked by wolfman and they were survived because Bill hit him with an arrow (I don't remember if it is an arrow or some other weapon but he hit wolfman in his eye and that's confirm) and run for their life on silver(bill's bicycle) . Wolfman chase them for sometime than just gave away.

3.Mike Hanlon - I don't remember if he had real confrontation with IT

BUT

He is attacked by Mighty bird (May be this event will be included in squeal)

4.Eddie Kaspbrak (Attacked by leper) - Eddie attacked by leper in neibolt street house but he also able to escaped this attack.

5.Stanley Uris - He is attacked near Derry light house by IT. He able to survive this attack

BECAUSE

He just speak names of several birds and IT just taken aback. For some reason IT fear birds or don't like birds and this gives stan some time to run (In book IT hadn't been clarify why IT is afraid of birds though . I tried to search the reason but in vain.)

6.Beverly Marsh (Bathroom) - Yes in book we find this scene which is very well described and which is the best scene in movie I found and bev survive just because of adult interruption(His father).

All these attacks are not like teasing or something from IT(1986 novel). These all are severe attacks by IT and All seven survived these attacks are mere luck(In case of Ben,Eddie,Beverly),some bravery(In case of Bill and Richie) and some wit(In case of Stan).

In movie there is a constraint of time that is why most of this incidents are not included.

As you say, It feeds on their fears. It wants them to know It's here chasing them. I think that's why It doesn't kill them the first time around, It wants to scare the shit out of The Losers, not just kill them. I don't think it's got anything to do with the Turtle, Since It's feeding on their fear, It throws oil on the fire.

First, let me present some information about IT. IT would sleep for approximately 27 to 30 years at a time, then awaken to feed (primarly) on children. IT also finds scaring children comparable to salting one's meal:

IT is a monster of unknown origin that preys on Derry's children and humans every twenty-seven years. [...] IT finds the fear in children akin to "salt(ing) the meat".^Wikipedia

Now, let's answer the questions:

Why did IT not just kill them?

In the movie, IT's preying season (that happens each 27 years) apparently started around October 1988, when he killed Georgie Denbrough (Bill's younger brother). We can see that IT killed Georgie right away without it previously scaring him (the basement scene prior to that doesn't count). So apprently IT kills at the begining of its preying season just to fill its needs and satisfy its hunger, similar to a hungry person who just eats anything without carring whether it is delicious or not. But towards the end of the preying season (next summer), he starts scaring children (salting its meals), because at this point, why not?

Were they just lucky to escape?

Yes and no. They were lucky not to encounter IT at the begining of its preying season, because then they may just have been killed immediately to satisfy its hunger. That being said, they (or at least most of them) also experienced some very harsh events prior to encountering IT who is now no longer killing children right away, but scare them for enjoyment. For example, Bill suffered the loss of his younger brother, and he is determined to do whatever it takes to find him (or his corpse), he even decided to go into the scariest house in town alone to achieve that. And Beverly and her abusing father. All this reduced IT's scaring effects on the group, specially on Beverly, who was the first one to turn the tables on IT. At one point she said to IT:

I'm not afraid of you.

IT underestimating the group as being ordinary children (who haven't experienced any harsh events), made him screw up the scaring process which gave the group the idea that they could actually defeat him if they get over their fears and be brave, which we saw building up throughout the movie little by little.

Also the group being a group and not separate individual?s is an added point that makes the scaring effects even more reduced. It is true that IT, at times, scared each member of the group individually, but once they got together and talked about it, the effects got reduced.

Why didn't IT just kill them after it knew they couldn't be scared?

Well, at the end of the movie, it tries, but it's too late now, because they are not affraid of it anymore, so they fought back and IT was the one who got scared and eventually fell down the well.