Having just done a modual on theis subject at uni i feel informed to express to you why some books are changed to film most of the time it is the directors view they may take a story and change it to the piont that it donst resemable anything like you imagieanded and or they take most of the story and leave things out or if they choose to be evry close to the book then we has readers of the said tend to find fault as always everyones imagainatation is diffenert and so each person has their own view how a chacter sholud look like when a director leaves things out he is chossing to transform the oringal format to his choosing to express many things not just recreating the story

Geoffrey Wagner suggests there are 3 types of dramatation from book to film

Transpostion from a book to film will little change
commentary from book to film which is purposely altered
analogy from book to film where the so much has changed that to the avergae viewer it like watching a new story like that of (alcospele now film is based on heart of darkness book )
i loved reading atonement but also loved the film
being dislexic i perfer watching films then i read the book