For me, it goes back to poltergiest, when the boy is laying in bed and the creepy clown doll is flashed in the lightening, and then it's gone. Next sceen, it's under his bed, and pulls him under.
Then there's the whole thing about OBSCURING your true face and emotion. *shudder* Yeah I can dress up, but clowns just aren't my favorite thing. They creep me out. "IT" really sent the unease over the edge.