I talked about this with a good friend in the movie business lately, and she told me that it's the good old "worked once, will work again" thing. Everybody is playing it save and doesn't WANT to make creative new stuff as long as there is the risk that it could net a loss. So you just make movies about things already successful, since there IS a fanbase already, so you'll have a guaranteed income. Save. Secure. And ultimatively boring ...