Warner Bros didn't want to make the Hobbit. They wanted to make another Lord of the Rings movie, and had to use the Hobbit for it. The Hobbit is very much NOT a Lord of the Rings story, despite the shared setting. Square book, round movie.

Also, they knew there wasn't enough content, but Warner Bros had to split the profits of the first movie five ways. They didn't have to do that for the second movie, and then they added a third to squeeze out even more.