Texas Chainsaw Massacre, Jaws, Halloween, Shivers, Rabid, and Alien are from the 70s. The 80s didn't set trends in horror, they followed them. Modern Body Horror, Modern Slashers, Blockbuster Horror and Modern US-type Monster Horror all started in the 70s.
This is not to say that Friday the 13th, Nightmare on Elm Street, Scanners, Predator, etc are not good, but they represent a sort of maturation of themes that were already there. I am not going to get involved in an argument about when Cronenberg or Carpenter were good, but they became players in the 70s and did a lot of famous work in the 80s.
As for why, well part of it was Vietnam vets. Texas Chainsaw Massacre was largely done by Vietnam draftees. You had a whole lot of people who knew what it looked like when a dude gets disemboweled, understood what it feels like to feel hunted and be a hunter, and a nihilistic philosophy where you just sort of sit back and watch someone blow their brains out because its a good show.
Another part was that film creation and distribution started becoming cheap. A lot of people in film get their start in horror because corn syrup, red food coloring and condoms cost pennies. The introduction of enthusiastic, cheaply equipped amateurs to film in the late 60s to early 80s was like the introduction of enthusiastic, cheaply equipped amateurs to politics, that is to say, a lot of very interesting terrorism happened to the public. Most of it was misguided, but ideas that would otherwise have never been welcomed into the mainstream are forced there on a wave of cheap bloodshed and cheerful violence.