They mainly changed a lot of things for modern audiences to accept it since a lot of it was for purposes of dramatic tension and to answer questions like "well why doesn't he just x." A lot of things happen over a long period of time, too, so they condensed stuff to go at a movie pace.
I loved the films, and understand a lot of what they did to change it for film and update it for people fifty years after it was written, but it is quite different in a lot of ways, not all of them good.