Aside from The Exorcist and Rosemary’s Baby, there were a ton of lesser-known/B-movies about Satanic cults in the 70s - I’ve seen quite a few on AMC recently (The Devil’s Rain, To The Devil A Daughter, and The Touch of Satan are some titles I was able to find). What was going on in the culture that resulted in such a seemingly bizarre (for such a “God-fearing” nation) subgenre? Was it leftover anxiety from the Manson murders? A reaction against the aforementioned religiosity of America? Or just cheap attempts to reap some of the profits of The Exorcist and Rosemary’s Baby? It just seems weird to me, since the whole idea of devil-worshipping cults has been pretty much extinct in film since then.
Same reason there were so many disaster movies in the 70s: One or two make a hit, and then a fad starts.
Also, perhaps, because in the 1970s, following the enthusiastic spiritual experimentations of the 1960s, and the popularity of the New Age and paranoramal stuff, it was gradually entering American public consciousness that real-life “witch-cults” existed – but not necessarily that there was any important distinction (many still would deny there is any distinction) between Wiccans and other neopagans on the one hand and the Church of Satan, etc., on the other. “Witch” still connoted Gothic witchcraft, what was supposed to exist in the 16th Century, i.e., pacts-with-the-Devil stuff. And, of course, the latter dovetails neatly with “End Times” movies such as The Omen, which started to become popular in the same period. I would say that is attributable to the rising visibility of Evangelical Protestants such as Hal Lindsey, propagating their interpretation of Bible prophecy, as well as to a widespread perception in the 1970s that American society was in deep decay.
Add The Mephisto Waltz (1971) and Race with the Devil (1975).
There was also an outburst of Christian-themed works in pop culture at the same time:Jesus Christ Superstar and Godspell, Top Ten songs like Oh Happy Day, The Lord’s Prayer, and Spirit in the Sky.
Good question and I’m sure BrainGlutton is right about a fad being set off. More generally I’ll point out that people are always attracted to things they are afraid of as a source of entertainment. Fear of Satanism sure didn’t die off in the '80s even if there weren’t too many movies about it. In fact, among the gifts the '80s gave us were the “Satanic panic,” or Satanic ritual abuse.
By the way, if you like these movies, try Satan’s School for Girls (1973). It’s hilarious.
On that note, check out The Big Book of the 70s. Covers the sex revolution, feminism, drugs, disco, Patty Hearst, the Bay City Rollers, Howard Cosell, Mohammed Ali . . . all the signs of the End Times! 
This pretty much. Remember that the books were written in the late '60s/ early '70s on the heels of sensational reports of counter-culture/ new age fascination with all sorts of things ranging from neopaganism to outright Satan worship. Then you have the book-to-film lag time, and then you have the posers buying into fads five years after they were first cutting edge.
I agree that profit was the motive, but part of the reason that the genre was so successful was that some of the stories were purported to be based on real events (The Exorcist and The Amityville Horror come to mind). So in addition to the books and films themselves, there was an ongoing investigation of the claims in the books, both by fans and in the media, e.g., in some magazines and on shows like In Search Of.
Because Hollywood hates originality.
Next question.
You do know who was president during the first four years of the decade?
The book Rosemary’s Baby was a big 1967 best-seller, and the film came out in 1968, both preceding Manson. I actually suspect the big growth in Satanic movies came from the brilliant publicity used to market The Exorcist (“get yer vomit bags right here in the lobby!”) and its subsequent success.
Not just movies. Fundies were finding what they believed were Satanic references in the lyrics of hundreds of rock-and-roll songs, and it was the era when “Satan worshiping” heavy metal groups first started to become predominant. Urban exploration sites in the area were associated with Satan worship, too; there were always stories about teenagers sacrificing black cats at some abandoned factory or grain mill.
Don’t forget The Sentinel.
Not to mention a burst of Satanic/occult-themed works like Sympathy for the Devil The Crazy World of Arthur Brown’s Fire and almost anything by Black Sabbath.
Ditto, with vampires for the 90s and 00s. But I’d extend it to books.
You also had the end of Hays code which allowed more of these types of movies to be made. Plus some of them such as “Night of the Living Dead” sold a lot of tickets, no doubt aided by an article in “Reader’s Digest” slamming it. I know I wanted to see it after reading that article. But as others have said, once a hit hit happens, you can count on Hollywood making 100 films on the same subject.
One of the things that makes me happy about my little black kitten at home - I hate to think about what might’ve happened to her littermates. I wish I could’ve adopted them all.
Back on subject, it was around this time a lot of metal groups really played up the satanic angle of their posturing, just so the kids would be attracted to them to freak out Mom and Dad.
Sort of a hijack, but I recall reading that two ex-cops made a god living, by going around to parent’s groups and warning them of the dangers of “Teen Satan Worship”-whatever that was!
The fundys really went crazy-they saw satan around every corner.
One thing I could never figure out…if satan was so gol-danged powerful, why was he capable of being controlled by teenagers chanting fron a magic circle?
Dont forget The Omen I & II
FWIW, I have a book about Baby Boomers and Generation X which suggests that this comes from the oldest Baby Boomers starting to have kids and resenting that they’re not the center of attention anymore, coinciding with books like “The Population Bomb” frightening people into being hostile against people with young children.
Just throwing that out there.
This was the time when thousands of people claimed to have been tortured by “satanic cults” and become “multiple personalitys.” To use an unfavorite phrase, the whole thing was a stupid witch hunt caused by mass hysteria.