So far, only three
Kurt Vonnegut, Jr. novels have been made into
movies:
There are probably many reasons why this is true. The most important is that Vonnegut's
nonlinear narrative style does not lend itself easily to a
screenplay--but his most nonlinear book,
Slaughterhouse Five, was done as a movie, so that may not be true.
I think another reason is that his stories depend a lot on word choice and sentence structure and his postmodern prose which never translate to the big screen. Many of his novels, while great stories, really get their emotional impact by how they're put together on the printed page.
Interesting fact is that Nick Nolte has starred in 2/3 of all the movies made from Vonnegut novels.