B/c. they want to create an original story set in Tolkien's world. That spells PHAIL, in my books. No tinkering on the behalf of Peter Jackson (et al.) could make it work.
BTW: Hopefully that part of it won't happen....again, I don't really give if they make "The Hobbit" into a film. But nothing else. PLEASE. If they try to, well, I hope that The Estate goes after them.