Are you asking if anyone can tell you why Gary Kurtz isn't in the prequels? I remember someone posted a thread recently that directed to an interview with him. Long story short, though, by the time they got to Return, Gary didn't like the direction that George was taking the movies. He felt that George was more interested in action than story, didn't agree with things like the second Death Star. They fought a lot, so they just decided to go their separate ways.