My guess is that New Line saw the receipts for their disastrous Golden Compass and figured they better settle up with Jackson quick or there won’t be a studio in ten years.
There are certain problems with The Hobbit as a film property. First, there are no women in the book, no female characters at all. Second, what characters there are are mostly a gaggle of largely indistinguishable dwarves. Third, the central element that ties in to The Lord of the Rings happens less than halfway through the book.