Something evil is haunting the town of Derry, Maine. It’s a clown that really likes fucking with the protagonists. Can Jon overcome his love of the original Tim Curry It? Can Jef overcome the fact that he hates horror movies and movies in general? Only one way to find out.
System Mastery 
The reason adults in IT can’t see Pennywise is more or less the point. IT is a metaphor for cycles of abuse, and as a result adults ignore it just like systemic abuse in real life.
Are you being facetious with the “how did Cary Fukunaga get atttched to this?” comment? Like you guys don’t know he made a name for himself with True Detective, with it’s creepy supernatural-connected death cult, which seems like a natural fit with the source material?