As per this website's Friday post, anyways. The real question: Um, why? Like two or three things total happen in that book! Total, finito, that's it. Those are the only plot developments, and they do not require two movies! Do not let the length of the book fool you, people, nothing happens in the ridiculous number of pages we are presented with.
Gah!
(and yes, I know the answer is 'because they can make more money this way,' but nonetheless