It doesn't have to be mysterious to require some dramatic explanation why people didn't like the Prequel Trilogy. The whole point of making the PT was to fulfill the promise to fans to create the "backstory" to the Original Trilogy, a set of sci fi films beloved by millions for decades. Plenty of fans just didn't find them as compelling, that they didn't live up to the promise. That's that.
There was a long standing myth (somewhat helped by Lucas himself) that the Prequels were not going to be brand new stories, but merely "revealing" what he had "always intended" and planned all along since the beginning. It turns out, and there's monumental proof of this, that he did "make it up as he went along."
Maybe some people didn't like it because they've made Star Wars into a religion instead of just fandom of a couple fun movies. Maybe others didn't like it because they were wanting to backlash against something popular. But I think you should give the benefit of the doubt to anyone who disliked them.
People don't need a "reason" to dislike a movie. It's a piece of entertainment, which is subject to personal taste. But the primary argument in defense of the PT is that it's Star Wars, and the primary argument against it is that it's Star Wars. Maybe the expectations were unfairly high, but on the other hand, the whole reason to make them in the first place was based on the promise of more Star Wars magic. So by that logic, Lucas should never have tried to create them in the first place.