Well, for one thing, different people like different things.
Many people (including myself) think it's badly written, the plot it boring and very predictable, the characters are shallow and vapid, and there are probably many others.
It's also the fact that there are so many obsessed teenage girls who have nothing better to do with their life than obsess over FICTIONAL characters.
I myself read the books when they first came out, and have seen the movie. But it's lost pretty much every single ounce of its charm due to the fact that there's all these screaming fan girls who are giving other teenage girls a bad name.
Besides, there are plenty of other GOOD books out there, ones that are actually worth reading.
EDIT: Something else I just thought of. In my opinion, the books are also quite offending to women. They don't portray Bella as a strong girl, one who can actually think and do things for herself. Personally, I'd much rather read a book where the girl has a mind of her own and can stand up for herself. In which case, I'd highly recommend reading Graceling, a new book that came out by Kristin Cashore.
Avid reader, 14 on the 27th.