From an interview with Kathleen Kennedy about this:
Peter: Who and how is it decided that the treatments that George Lucas had submitted for the sequel trilogy were to be abandoned?
Kathleen Kennedy: Well, heres the thing. There are certain things they retained and obviously everything George created, you can imagine, every single person involved in this process hugely respects and wanted to know as much as they possibly could about the universe that he was describing. He had specific plot ideas that evolved. He sat down with me in 2012, and now its almost 2016, and you know the development process, where you bring writers on and once the story starts to take shape, it evolves. George wasnt a part of those development discussions, so it was a fairly natural process of evolution. It sounds like we ignored him but thats not really what happened.
Peter: How is J.J. Abrams final movie, The Force Awakens, different from what was planned early on?
Kathleen Kennedy: You mean from what George was talking about?
Peter: Yeah, like was Georges story more focused on the original trilogy characters?
Kathleen Kennedy: No, because the legacy characters play a significant role in this, and I would say and inside the balance that George was talking about to begin with. It was really much more to do with the specifics of the history of the saga. We changed the order of a few things, lets put it that way. We didnt make some wholesale change.
And legacy characters?