Its something that happens, every few years the horror film industry develops an obsession with a sub-genre.
The 30s to the mid 1950s, Monster films were the film du jour, with films ranging from vampires to werewolves to invisible men. The films were, as so often happens, milked to such a stage, that all that was left, was the comedy films.
The 50s and 60s saw two trends co-existing pretty well; the invaders from outer space and giant monsters were tearing cities apart. Both types were so successful, that they saw many cheap rip-offs which killed the genres for a decade or two.
Most of the 70s was filled with demonic films, from Rosemary's Baby to the Exorcist and the Omen. Again, the films were so popular that the genre was drowned in 10-minute productions.
A shorter lived genre in the latter half of the 70s was the animal horror films, most famously, Jaws. Cheap to make, but also way too easy in making them cheesy.
Of course, then there is the most famous trend, the Slashers of the 80s. Given the numbers of holidays that were in the calender, it's surprising that the population of North America wasn't wiped out by July. The sheer amount of sequels that these films inspired pretty much killed the genre by weight alone.
Horror films, perhaps more than other genre, really do need trends to help them make money. Which is surprising, because many of these trends tend to reflect the social mindset of the time. The problem comes from the over-dependence on these trends leading to cash cows that get milked pretty quickly.