cw8: Coz moar money, moar money, moar money.
Part of it is the same mentality that drove studios to devide the final books of Harry Potter, Twilight and The Hunger Games into two films.
The result with Harry Potter and the Deathly Hallows was possibly the most poorly-paced films of the series. The first film dragged on ceaselessly, while the second film dragged out the Battle of Hogwarts so long that all the carnage just became boring after a while.
Not seen Breaking Dawn myself (I could just about stomach part 1 and 3 because they had something resembling a decent story when Kristen Stewart and Robert Pattinson weren't moping over one another), but my better half - a sworn Twilight fan - has told me that both films drag on and are the worst of the series.
And I expect that the same problem will befall Mockingjay.
But it ultimately doesn't matter that this practice produces poor-quality films - the drooling masses swarm in and pay money for it, so they'll keep doing it.