Why Five Nights at Freddy's is Scary

Five Nights at Freddy's is undeniably scary. While it relies on jumpscares, notice that it uses them not to scare you along the way, but more to condition you for much greater and sustained fright and paranoia. FNAF is a clever, well-designed, simple horror game that in many ways has redefined horror games as a genre.

What exactly makes Five Nights at Freddy's so scary? What game mechanics does it employ to do this?