Vanity Fair has a new interview with director David Yates!

Why did you and Warners decide to split the last installment, Harry Potter and the Deathly Hallows into two parts?
Three reasons. One is it’s a huge book so there’s so much stuff in it. You go through the list of things you want to put in the movie and there’s so many huge set pieces that by the time you get to the end of it you kind of go, “Whoa this would be the most expensive movie of all time.” Far more expensive than any conventional blockbuster. You have to find a way of somehow keeping all the things you want to do, so it is easy to split it across two films. [Screenwriter] Steve Kloves was working on the adaptation as we were shooting Half-Blood Prince and he just rounded us up and said, “Look guys I am having a real problem stuffing this story into two-and-a-half hours. I think narratively this might suit two parts.” There was that. I get frustrated when fans say, ‘Why didn’t you put this in or put that in?’ So it was a combination of the adaptation process and the fact that I just want to give the audience everything we can from the book, basically.