I guess he wanted to tell that part of the story first. And the only reason anyone cares about the prequels is that we want to know how Anakin became Vader; they don't really have enough of their own plot to attract a crowd. They wouldn't fly on their own.
Do they usually release trailers before production has finished? And if so, they can't show too much footage surely...hmmm. If it is true, Emcee, you could always pay your $10 and leave after the trailers to sneak into something else!
I've never heard of them switching the trailers once a movie starts playing. I've always thought that the trailers stay the same throughout the run of the movie. Is this a rare occurance?