It is unclear why Cooper left. Sources say the parting was over creative differences, while one insider says it came down to the project’s potential rating. Cooper wants an R rating, in keeping with the rawness of King’s novel, while Warners, the lead on the project, aims to make a PG or PG-13 movie.
Another source said the project’s scope is so massive that the studios are not sure how many movies are needed to cover the entire adaptation.
Whatever the reason it must have been decide fast. Scott recently (during a recent press junket for "Out Of The Furnace") talked about it.
Thanks to Ari and Lou.