Why was Jaws in its day so effective at scaring people?

I just look at some of the movies today that area lot similiar, but with a lot better graphics, and we laugh at movies like that and they tank at the box office. What about Jaws made it such a clic. Even this new movie Primevil isnt supposed to be that good, and its about a real crocodile who has really killed over 300 people, if the truth isnt even scary anymore then what is.