In my opinion, books can be readily compared to cars: Things like Harry Potter, Twilight, and so on, are mass produced and appeal to a wide audience, like a Ford Fiesta or something like that. Meanwhile, a Pratchett novel is a bit like an antique Mercedes: It's built to excellent standards, and exceptionally rare, (It's so hard for me to find a Discworld book in a bookshop or local library).
The analogy breaks down when you get to 'why people don't like it'.
I think the most likely problem is that people expect 'fantasy' type fantasy, whereas Pratchett fantasy is totally unique, and some people just aren't prepared to accept change. I still maintain that the Discworld novels are the best thing I have ever read (Not just post-reading hype either), but some people are just too narrow-minded to accept something as wonderfully satirical, absurd and ingenius as the Discworld.
Phew, sorry about that! When I join a forum I have a habit of going around and posting in any subjects that interest me. :\