Imagine what happened in/with/to Derry and It in the Bad Future timeline..
Pennywise, the child eating Eldritch Abomination, knows that Jake Epping is from another Time-Line, and is aware of how to get to them. What if he decides that one Derry, Maine isn't enough?
In the front cover, what you see before reading the book, you have a newspaper article about JFK's killing in Dallas. In the back, what you see when you finish it, you have a newspaper article about an attempted murder on JFK in 11/22/63. Regarding whether if that's really what happens in the book or not, that's some very clever Fridge Logic artwork design.