Why did they decide to continue with the Skywalker Saga?

But which reason lead to the decision to continue with the Skywalker Saga instead of creating some other stories situated in another era of the galactic warfare?

A lot of this is covered in a Vanity Fair article.

Before the purchase by Disney, Lucasfilm had, in the words of Kathleen Kennedy..

“There were a lot of things going on in terms of the infrastructure of the company so that we could shift strategically from what had really become a licensing model back to a production model.”

Essentially, as explained in the article...

...in preparation for selling the company, which to some extent had been coasting on royalties from Star Wars toys and other merchandise, Lucas had decided to make more movies. He sketched out ideas for episodes VII, VIII, and IX, to be set initially several decades after Return of the Jedi, and approached Harrison Ford, Carrie Fisher, and Mark Hamill about re-upping.

But once the deal was done, Disney decided to go in a somewhat different direction.

As quoted by JJ Abrams

Lucas’s treatments had centered on very young characters—teenagers, Lucasfilm told me—which might have struck Disney executives as veering too close for comfort to The Phantom Menace and its 9-year-old Anakin Skywalker and 14-year-old Queen Amidala. “We’ve made some departures” from Lucas’s ideas, Kennedy conceded, but only in “exactly the way you would in any development process.”

Kennedy had assembled a story team and courted JJ Abrams to come on board.

He says...

“I fully expected to gratefully pass on this movie,” Abrams said, describing his meeting with Kennedy. But he grew intrigued when she told him the picture was a blank canvas. Implicit questions tantalized him—“this idea,” he said, “of what’s happened in these past 30-something years. Where is Han Solo? What happened to Leia? Is Luke alive? These questions started to percolate, and I found myself thrown completely by this visceral hunger to be part of this world.” He added, The logic of why it was the wrong thing was overruled for me by the emotion of it.”

Once a script outline was in place though Kennedy made the point..

Kennedy described the script’s mix of old and new characters in terms of audience expectations: “It’s sort of like going to a concert where you want to hear the new stuff that they’ve written, but really you want to hear some of the old songs. And we’re in a similar kind of thing: we’re getting the band back together, and we know that people are going to want to be reminded of the things they love, but they’re going to expect to have a new experience.”

The Skywalker Saga is a generational one for the Skywalker family. From Anakin to Luke or Leia and on to their children.

In summary, a combination of financial reasons, protecting and extending IP, and creative in giving the fans a familiar link to the old franchise and whilst at the same time as introducing new characters.

The presence of Luke Skywalker in the new trilogy doesn't mean that is based on the Skywalker's Saga.

We don't know how the story will evolve, maybe Luke Skywalker will be a supporting character, like Obi Wan Kenobi in the original trilogy (supporting, but important anyway).

In my opinion, they decided to take place the story after Return Of The Jedi because they want to involve the same characters, in this way they stimulates the audience.