Director Andy Muschietti explains that IT movie almost featured cameos from famous ’80s movie monsters in the movie:
Obviously we considered that for a bit, but I wasn’t too interested in bringing Freddy Krueger into the mix. I love the story and I love how Stephen King basically makes a portrait of childhood in the ’50s. He’s very genuine when he brings all the Universal Monsters to the repertoire of incarnation because that’s what kids were afraid of. It would be a natural path to try to recreate that in the ’80s, but I really wasn’t too crazy about bringing stuff like Freddy Krueger into the story. I thought it was a bit too meta with New Line involved in the film. It’s distracting and it didn’t feel right, for some reason.
I wanted to bring fears that were a little more layered and related to childhood trauma and more surprising in general. I think that Stephen King was open to that. When he saw the film I basically wrote a letter to him asking him for forgiveness for having taken so many licenses, especially with the many different incarnations of Pennywise. He said “Don’t worry about it. All the changes are great!”
Source: Ain’t It Cool News