However, much like the original Tommy Lee Wallace-helmed TV version, there was still a bit of a battle over what was included, and what would be cut, from the story that makes up IT in its totality. Ultimately, with two films and roughly 5-6 hours of time to tell the story of an over 1000-page doorstop, sacrifices must be made.
Article source: https://www.cinemablend.com/news/2480345/why-it-should-have-been-a-trilogy-and-how-that-wouldve-worked