THE WEIRDIE FILES

 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.

So in Evanier's book KIRBY, ME claims, maybe a little dubiously, that when Kinney Corp bought DC in 1967, they thought they were getting the top company, only to become displeased when they learned that Marvel was such a strong second. (I think Roy Thomas claimed Marvel didn't obtain the majority market share until the early seventies though.) Still, that story isn't absolutely necessary to put across the notion that someone in management thought it was time for some changes. Infantino was made first art director and then editorial director in 1966 and 1967, and it looks like promoting horror and the Gothic was his major "experiment." Not only did he get rid of the superheroes in HOUSE OF MYSTERY in '67, he also debuted DEADMAN in the failing book STRANGE ADVENTURES. The Spectre had been revived earlier under the tutelage of Julie Schwartz, but the initial format was so rationalized that any "weirdie" appeal of the hero was nullified. Spectre also got his own title in 1967, and though it didn't last long it soon converted into spookier stories before it died. In the late sixties and early seventies, even some of the "mainstream" DC superheroes began exploiting Gothic/horror themes on their covers, such as (obviously) BATMAN but also less obvious types like FLASH and TEEN TITANS. 

One fan attributed the big change to the influence of DARK SHADOWS in '66, but I think it was more likely that DC saw that the Warren magazines had been doing well since 1964 (EERIE) and 1966 (CREEPY) respectively, and that they hired guys like EC stalwart Joe Orlando to cut into that action. That also probably led to the revival of The Phantom Stranger in 1969, as well as another fifties character, Doctor Thirteen. The intersection of the two seems to be the first regular convocation of two "weirdies" at DC Comics, in 1969's SHOWCASE #80-- though the good doctor was dropped from the Stranger's adventures pretty quickly.