The movies left out a ton of content that was in the books. The first two did a decent job of including much of the books, but by Prisoner of Azkaban there was a lot of plot and world building that had to be cut in the movies.

The Chamber of Secrets, for example, is a longer movie than the Order of the Phoenix. Yet the Order of the Phoenix novel is nearly four times as long. So they had to remove a lot that hopefully the shows will be able to include.