A two-hour movie can't support all of the subplots and characters that a 500-page book can. A reader might spend 10 hours reading that book. That's a lot more time to convey information. Here's an easy example: Go read any of the Harry Potter books and then watch the films. You'll find that many of your favorite scenes and characters from the books won't be in the films. A film just can't handle the level of complexity a book can.