He couldn't write an interesting Episode 1, so he started with the story already running, and instead presented what would appear to be an interesting episode of what seemed to be a much larger story. Which was very cool, and left lots of room for people to fill in whatever details for themselves , or speculate what had happened before we got to see Vader board Leia's ship. The mistake he made was going back and trying to fill in the story holes without what would seem to be even a casual fans knowledge of the films Star Wars, ESB and ROTJ. Instead he used total ownership of the franchise to bitch-slap everyone around (including the story) to try to fit his visual fantasies onto the screen.
Personally, I wish he had just made some new piece of crap to do that with, and leave SW alone.