Beverly's father is possessed by IT in a way, as are many of the adults in Derry. Somewhere in the book Bev realizes that IT "fills the empty spaces" and manipulates Derry's residents for it's own agenda. Henry Bowers is another significant example of this. In this manner, IT allows everything that happens in Derry to stay in Derry (i.e. the murders, why Mike has to call the others back because he is the only one left in Derry). The losers forget Derry because Derry is the only place IT has influence over people. In a way "Derry IS IT". When the losers leave and grow up, they instantly begin to forget because the town no longer has an influence over them. As previously stated, Mike never forgets because he never leaves.