People want to be disturbed when they go see a horror movie.
'Eraserhead' is a weird, horrible nightmare, and it doesn't narratively make sense. Stuff's happening, but you honestly feel like you're in a nightmare, and it has such disturbing imagery that it stays with you forever once you've seen it.
I think that horror films have a very direct relationship to the time in which they're made. The films that really strike a film with the public are very often reflecting something that everyone, consciously or unconsciously feeling - atomic age, post 9-11, post Iraq war; it's hard to predict what people are going to be afraid of.