Because 2 movies = more $$$, especially since this film is the last chance to milk a Harry Potter film. At least for now. Good thing there's laws against making up your own Harry Potter stories for publication.
I don't think they will cut so many good bits out if the movie because its 2 parts....come to think of it they should have done that with more of the movies.....
well since they cut out ALOT of information from the 5th harry potter movie they have to explain everything in the 7th movie. and that would take a long time to explain everything, so i think that is why they are splitting the 7th book into 2 movies. and i dont blame them. if they only did one movie for the 7th book, you would be screwed, you would have no idea why something happened or why this person died. that is why they split it into two movies. the books explain everything and since some ppl dont read the book tht is why they r splitting it.