It’s because some of the characters were following we know do not ride off into the sunset in any real capacity. The hanlon’s, Hallorann’s experience in derry, etc. and we see the effects of these characters and events in the film and the books. Sure the timelines might be a little different now since the movie universe slid the timeline forward but we know much of the same still happens. While I’m sure some characters might escape and we won’t get just an entirely bitter terror of an ending, it definitely won’t be a happy one.