It's all about money. Warner Brothers hit the mother-load with The Dark Knight and they know that HBP is a sure thing. They don't need two mega-revenue-generating films in the same year.......especially when they know they don't have a blockbuster on their slate for next summer. They saw an "unfulfilled market" opportunity for July 2009, so they pounced on it. Now they'll have two consecutive summer hits.
Don't be fooled. Some may try to suggest that the movie is flawed and needs a bunch of work and that changes have to be made. Post production is complete, they aren't doing re-shoots, it's done and in the can, and they are moving forward with Deathly Hallows. This is all about money, and sadly, its at the expense of the fans.