Because the world is NOT perfect, and not everybody agrees on conditions. It's easy to say, "everyone should have been on board, and should have agreed", but that does not work. I don't know the fine details behind the production of the Hobbit, but clearly people were NOT in agreement, and could NOT agree on the terms of production. There was also a lot of shuffling with actors Ian McKellen and Hugo Weaving. Had something to do with the SAG (an actors' union), and the fact that since they're both members, they couldn't work for a movie in a country without the guild presence (not sure how this issue was resolved; that happened I think over a year ago). In fact, I think the main reason why it took 5 years to make Dawn Treader (okay, not to make, but there was a 5 year gap) is because of production problems and legal issues with Fox/Disney/Walden.
And obviously, the ones connected with Twilight were in agreement over everything in production, which doesn't happen very often at all. I am kind of wondering if perhaps they also delayed Hobbit's production because of 2 franchises coming to an end, Harry Potter and Twilight. Remember, Breaking Dawn part 2 is coming out next November, a month before Hobbit is released. I have a feeling that BD part 2 will still be a threat in the box office (though why these movies make so much money is beyond me; nothing fascinating about a bunch of demonic creatures and a rebellious teenager), so Hobbit will have to push hard to be on top. HP is no longer an issue since the franchise is over with.
But I personally am tired of this debate 'cause I dislike having to repeat myself more than once.
Note: something else I was thinking of the other day-could Hobbit, part 2 be years after Hobbit, part 1 (story-wise)? Is there the possibility that part 1 will convey most of the book, part 2 will wrap it up, and part 2 will also move forward several years? Doesn't make a lot of sense, but could happen, I think.