Why is it so easy to become king of Wakanda?

It's not THAT easy. But the process is their tradition. Remember, the ruler of Wakanda is also given the powers of the Black Panther and is expected to protect the country from outsiders etc - so combat skills are definitely a requirement.

However, when a King dies, the King's first born takes over - much like in any monarchy. The big difference here to most monarchies is that they invite open challenges from the tribes that make up Wakanda's people. Each tribe has to nominate a challenger if they wish to challenge for the throne. None of the tribes wishes to challenge T'Challa's claim except for M'Baku of the Jabari tribe.

Killmonger has the ability to challenge for the throne because he is "of royal blood" - he's the son of T'Challa's uncle, who was King T'Chaka's brother. If he wasn't of royal blood, then his challenge couldn't be accepted and would be deemed invalid.

Conor McGregor couldn't take a shot at it as he's not Wakandan, so he couldn't be nominated by a tribe, and unless he could prove some royal connection to T'Challa, he's an invalid challenge as well.

I think the answer is a much simpler one implied by the film: it's a tradition that has become a formality.

At T'Challa's coronation, the three other tribes all presented a champion, but then declined to actually make a challenge. Everyone seemed surprised that the Jabari showed up at all—presumably the Jabari tribe had stopped attending the coronations long ago, or never had attended at all, just another facet of their separation from the rest of Wakandan society.

It seemed to be the case that no one was actually expecting T'Challa to face a challenge, and it's possible that there had been no challenges at all in recent history.

Well, for starters, strength is one the key point, probably the defining point, in selecting a king or a ruler.

Take any tribe for example, the ruler is selected by either a duel (displaying the strength of the person) or going out for a hunt (again, strength). And it's not just the strength that decides the ruler. In any fight or hunt, the person does display his strength to win, but it also requires his smartness, attention to details.

Take any fight from history, person at both side are strong (if not equally strong), but always, the person who uses his mind along with his strength only wins.

And so is the case in Wakanda too. The ruler from the tribes is selected based on the duel they participate in. And don't forget, before the duel, the person (or rather the one who is declared candidate for king, eg. T'challa) is made a regular person, with the superpowers.

Also talking about the 'giving superpower to both the candidate', it isn't the right approach as the flowers that give the power are rare and needs to be preserved.

And, if you think about it, Wakanda is just another place with different tribes, with some vibranium to their disposal ;) So why should they have any different ritual (one containing superpowers)?

You can find such rituals here -

African tribes' rituals

Terrifying rituals

I would consider becoming king of Wakanda easy...

Maybe i missed something, but during ceremony, only champions of each tribe and members of royal family could challenge king.

So if Connor MacGregor decided to challenge king of Wakanda he could refuse. Bear in mind Killmonger was not some stranger who came to Wakanda by accident. He was a member of royal family. Which gives him right to challenge T'Challa.

The King and the Black Panther are normally the same person but one can choose to give up the duties of the Black Panther and devote all their time to being King, which is what T’Chaka did. In his old age he passed the mantle of BP to T’Challa while retaining the mantle of King. So T’Challa ingested the herb but didn’t go through the burial process and transportation to the Ancestral Plane because he wasn’t taking the mantle of King. Because T’Challa was T’Chaka’s firstborn he was next in line to be king anyway so there was no challenge necessary. Also the king can deny any challenge issued as soon as they become king. So T’Chaka would have just said no to Killmonger if he challenged him.