DC jumped feet first into the supernatural/Gothic thing after having generally avoided that type of story for over 20 years, and it seems likely that Carmine Infantino was the biggest influence, as he himself claims in a JOURNAL interview:
I was trying to prepare for the inevitable. In my mind, “What if these things die? What if we’re back in the old days and suddenly superheroes drop off?” The reason I threw out a mess of different titles was, I wanted to sneak in The House of Mystery and The House of Secrets without people much realizing what was going on. Which I did. And also we had a chain of them out there, if you remember, and they were all successful before anyone at Marvel realized what was going on. So we had those going for us, and the superheroes going for us. Meanwhile I kept experimenting with different things.