It's one half Lucas' fault, one half the audiences, IMO.
He is a terrible director, but an excellent producer. If Irvin Kershner hadn't been at the helm of The Empire Strikes Back, and the technological limitations (and money/budget of the original Star Wars) of the day hadn't reined Lucas in a bit, the first three would have stunk just as much as the prequel trilogy.
The other thing was is when the first Star Wars was being made Lucas wasn't "god", Hammil, Ford, and Fischer all just threw themselves into it. Alec Guinness was the revered actor and whatever he did was considered golden, they and the rest of the crew was able to give him pushback and grief when he went off the deep-end. Cristensen and Portman and McGregor were reasonably big names, but Lucas, by that point is the biggest thing in Hollywood, bar none. If he says, "Deliver that in a dead monotone like it's a bunch of housewives on Valium reading the TV Guide aloud on Public Access Cable in Topeka Kansas at 3am..." you do it, no questions asked.
The other factor of course was a bunch of Gen-X late twenty and thirty-somethings thinking the prequel trilogy would wow them as much as the first one's did when they were four years old, then nine, and then thirteen. Your level of taste is a bit different then...
Honestly, if they'd started with "Attack of the Clones", and made the third movie between "Revenge of the Sith" and "A New Hope", and never had Midichlorians or Jar-Jar, and Portman and Christensen were replaced with actors who'd not take Lucas' direction literally, the prequel trilogy would have been mostly worthy.