Because the internet and, more importantly on this site, Yahtzee says they are.
Unfortunately, I don't actually listen to either on my movie decisions. Plus, being a big Star Wars fan, I'm a little more willing to overlook it's faults.
Same thing with Indiana Jones 4. I avoided internet discussion of it because people were already predicting how bad it would be over a year before it came out based on the stupidest things. (One argument was that there was a U.S. flag seen in a preview which offended some people terribly.) I still thought it would suck because Shia "Plague of the Transformers" LeBouf was in it.
But, y'know what? I really dug on Indiana Jones 4. I even liked Shia....somewhat or, at least, didn't hate him.
Same thing with video game movies. Gamers are absolutely the worst people to listen to when it comes to movie adaptions of games. The level of butthurt from Silent Hill fans when it was announced that the lead was going to be Radha Mitchell as opposed to a man could power the Eastern Seaboard for two years if we could have converted it to electrical energy. Everytime a video game movie comes out, the same level of butthurt rises from the fans because a character used a square key instead of a circular key to open a door or other such nitpicky nonsense.
So, in the end, you have to realize that the internet is a hate machine full of people who like Moviebob or think that "Over 9000" references is the peak of comedy. This could lead you to the conclusion that listening to people on the internet over what you should and should not like is asinine. However, while that is not wholly incorrect, the crowd can sometimes be right. So here is the best way to figure out if you should listen to arguments against a movie.
1) Do the arguments have the appearance of just hating on the movie because it's cool?
2) Are the arguments against it somewhat nitpicky?
3) Does the person write in all caps/like a 13-year-old badly in need of spellcheck/make ridiculous comparisons such as comparing a movie to the Bataan Death March?
4) Is the argument on IMDB?
If so, then ignore them. These people suck.
If, however, the arguments seem reasonable, then you may want to listen to them.
Getting back to the prequels, yes, they have their problems. As an example, there was only one short scene in Episode 2 where I actually bought into Anakin & Padme's relationship and the rest of their "loving" dialogue for the trilogy fell flat on it's cardboard face. But, while they have their issues, they are not terrible movies. It's just that the original trilogy had the benefit of not being torn apart by the internet before they were released in the theater plus people were free to make up their own minds about it rather than be confronted by a large hivemind that bombards you with how badly they suck/you suck if you like it.
Frankly, I've gotten into the habit of avoiding any internet discussion of movies that I'm going to see. This way, I can see the movie with as close to an optimistic blank slate as you can get. I'd rather be pleased/disappointed on the movies merits/faults on my own rather than going into it with the preconception surviving a nuclear blast by hiding in a fridge is the W0R5T M0M3NT 1N F1LM HESTOY 3VAR!!1!
So, in the movie, if you like the prequels, more power to you.