Films can't easily render the complexity of a good novel. I think the ratio is one page of text per minute of film, which makes the average film the equivalent of a 90-page short story. A TV miniseries can do more - four hours is about 240 pages, a short novel. A six hour series gets it up to 360 pages, a respectable book without much fluff. Any longer and you are venturing into TV-Season territory.
Then there are the problems of portraying the characters' inner lives - thoughts and opinions, old memories etc. Books are a more forgiving medium.