Lucas is only one man trying to do the job of four, producer, writer, director, and editor, he has a lot of talent but really tries to do too much. The reason he did well with American Grafiti is because it wasn't as nearly involved as any of the Star Wars films, he didn't have to go and found his own special effects studio for that film, didn't have to work with a huge multi studio production biking between sets while desperately trying to keep the studio from pulling the plug on the whole project!
Lucas didn't have time to do a good job writing the new trilogy, didn't have enough time to film multiple cuts of the same scene to fine tune the acting, and certainly didn't give himself the time to edit the films in a way that would lead to better passing, look at Episode II and the scenes he put in and left out, look at all the crazy action in the very long duel between Obi-Wan and Vader in Episode III, that crazy scene with them flying around from cables over a burning river of molten lava, it got to be ridiculous, I mean what was he thinking!
Better for Lucas to try and focus with doing more with less jobs, letting others take on some of the story telling burdens.
