Presumably, after Peter Jackson saw the final Harry Potter, Twilight, and Hunger Games movies being split into two movies, he said to himself, “I will one-up them all! I’ll turn The Hobbit into three movies!” And then I’m guessing he laughed an evil laugh while everyone pretended that he never made Meet the Feebles. Anyway, since absolutely everyone thinks that paying three times the amount to watch a movie is kind of idiotic, here’s the explanation as to why it will be a trilogy, via HuffPo:
The trifurcation has even upset diehard fans of the Lord of the Rings series, who you might think would be eager to get as much Tolkien as they can. Take Corey Olsen, author of the forthcoming book Exploring J.R.R. Tolkien’s The Hobbit. He’s been defending Jackson’s decision to make The Hobbit into two movies for months. “The thing that people don’t realize or think about is that there’s much more material than just the pages of The Hobbit,” Olsen told the Huffington Post. “They’re not stretching two films out of the 200 pages of the 1937 children’s book and adding their own stuff. They’re drawing on work that Tolkien did over the course of decades.” Olsen cited a great deal of Hobbit-related material in the appendices of The Return of the King and the late short story The Quest for Erebor as fruitful sources for Jackson’s movies, and ample justification for the two films that had originally been planned. But Olsen said he was less confident that there was enough to make three movies.
Oh, wait, it turns out Olsen isn’t actually affiliated with the movie itself, and is just writing a book about The Hobbit! Okay, so basically he’s just nosing in for the sake of publicity. Gotcha. So basically, the real reason for splitting the book into three movies is “MONEY. ALL THE MONEY.” Case closed.