Why is the movie always so different from the book? I'm almost too apprehensive of that, especially when handing my books over to a movie producer. However, I do think books adapted for the big screen do get more exposure.
Time is the key element - script writers have to condense 300 to 450 pages of a book manuscript into 100-120 pages of screenplay (and half of that is scene description). But, also, somethings don't translate well to visual presentation - the most noted examples being inner thoughts and emotional descriptions. Add to that the strong personalities involved in the industry and you have got some control issues.
I don't remember the author's name but an excellent book on the subject is THE BIG PICTURE.