Obligatory PA reference in 3...2...1...
http://www.penny-arcade.com/comic/2004/9/22/
See, I think that the real reason the prequels flopped was because of fan expecations.
Seriously, all Star Wars movies gave us the following ingredients:
Land-bound buildup (usually involving a chase scene, but not always)
Lightsaber duel
Epic space battle.
Good guys win by a whisker and we all go home happy.
... all backed by an epic orchestral soundtrack.
Now occassionally, they mixed up that story, or they dropped one element, but that's essentially it... and it's not a bad thing. Why? Because it worked!
No the real problem is that George Lucas, despite his pretentions to such, cannot direct a movie to save his soul! It's not simple coincidence that the most
critically acclaimed Star Wars movie was the one he didn't direct.
Now, the prequels might be worthy of additional excoriation given some of the retconning and the blatant attempts to exploit the child market (Jar-Jar), but overall, you have to say that he successfully identified what people liked and made an honest attempt to give it to them.
Ultimately, it really comes down to that people had built the SW stories into a myth-system, and Lucas really saw the plots a merely a framework to offer those experiences he thought would please the audience (since that should in theory be the most profitable-- and if you don't think this was about profits, I suggest you go to a local Toys 'R' Us and check out how much Star Wars merchandise is on the shelves even today), as cynical as that train of thought might be.