I can think of a few TV shows that stem from books: True Blood (from Charlaine Harris' Sookie Stackhouse series), The Vampire Diaries (books of same name), Pretty Little Liars (books of same name), Gossip Girl (books of same name), Bones (from Kathy Reich's Temperance Brennan series - this one's not based on each book exactly, but the show has its roots in the book series), Game of Thrones (from George R.R. Martin's series of the same name), etc. Pride and Prejudice has also been adapted into a tv mini-series.
I think there are several reasons why books are usually adapted to movies, rather than tv shows:
- TV shows are usually expected to go on for many seasons. If you're going with one book = one season, there better be a lot of books in the series, or else the tv show's creators, producers, writers, etc, will have to make up their own way of carrying on the show. This could alienate fans of the books. The only alternative would be to end the show after whatever number of seasons (which has to happen eventually, I know, but think about it - if they're basing a show on a trilogy, the show's only going to last 3 seasons. Not long.)
- Dragging out one book over an entire season can end up being boring. Reading a typical book, you get all the action in maybe 5 hours at most. Over the span of a tv season, you're getting the same info spread out over 16+ hours. It could get boring, and if the show can't hold the audience's attention, it's a poor choice to let it continue. That many hours is also a bit of a commitment: are you going to make sure you tune in to watch every single week? (A movie, on the other hand, is usually 2 hours - you get all the action right there, and there's not much time to be bored, and you probably can free up 2 hours easily.)
- In order to avoid boring the audience, or even to just make the story more adaptable to the medium, tv writers might have to make changes to the original book plot. As I said before, this could alienate fans of the book series.
- I think tv shows, in general, have smaller budgets than films. All the cool special effects could easily be lost. Also, as the previous answerer said, it's easier to make a profit from a film than a tv show.
Personally, I think that most books should just stay as books. I can't complain when a movie brings a wider audience to a book I love, but I think that books were written as books - and not as movies, tv shows, or anything else - for a reason.