Films and books are different mediums and film is much more constrained than literature. Film is a very specific way of telling stories. There are things you can do in film that you can’t do with other dramatic mediums – you can have a chase movie for instance, but The French Connection would make a pretty poor bit of theatre. By there same measure there are things that films can’t do (or can’t do well) so it shouldn’t be a surprise that something conceived as a novel doesn’t work well (or work as well) as a film.
That said a book can still be translated into a successful film, it just has to be re-configured to work as film, a direct transcription of a novel would be shit. But that means people who have already read the book are usually dissatisfied as bits they like have changed or disappeared. But that has to happen.
Change is part of the film making process – a film is authored three times (or four times if it started out as a novel). A screen play is written (or adapted) then its filmed – thats a translation of the script, and then its edited, and thats an translation of the filmed footage. A film takes 2-3 months to shoot but it can take a year or longer to edit and thats because the film is really made in the edit. The edit isn’t a process of making sure everything in the script is there as written and in the right order, its a process of constructing cinema and that can mean discarding pieces of script, even discarding whole characters that have been written and filmed. Usually 9/10ths of the way through you have to shoot new footage to fill cinematic gaps.
The other problem with adapting books is seeing any transcription of a work you know always disappoints. Fans of the Goon Show were disappointed when it was televised for instance, simply because they’d had a picture in their head of what the radio charactered looked like, so when the Goons moved to TV in their eyes everybody looked wrong. Seeing someones elses version of your ideas makes everything seem unauthentic.