They wanted to make a movie based on a book about revisiting childhood traumas, but they also wanted a whacky Evil Dead property.
It Chapter 2 also doesn't trust it's audience to be able to know when the clown is present hint
It's always and constantly gives it away instead of trying to scare the audience with tension.
This constant diffusion of tension is especially bad in scenes that are taken from the book. I don't need scenes to be the same as the source material. But when adapting a known property, there is a tension between what the audience is familiar with, and what the adaptation might try. For instance,
the scene where the old lady turns out to be Pennywise. This was in the book, but it's telegraphed with all the subtlety of a brick to the face in Bev and the lady's first interaction.
Some nitpicky things include Bev's abusive relationship which doesn't come back. Since it is irrelevant to the rest of the story, and to the developement of the character, why even have it?
The creative team involved set themselves an admittedly hard task as the adult portions of Stephen King's IT are much less interesting than the experiences of the children. I think the same holds true here.