Early bronze, DC horror has something else going for it above and beyond the horror genre - cover and interior artwork by some of the premiere artists of the day, at their peak. Specifically, there are lots of late silver/early bronze comic books collectors that collect anything by Adams, Steranko, Smith, Wrightson, Kaluta, the rest of "The Studio" artists, etc.,. That's personally what got me involved in horror (I collected Marvels based on character first, but as I "matured" I focused more on artists), and I pretty much only want the early bronze stuff. Kaluta did a lot of great horror covers after 1974 as well, but I'm not a big fan of the Dominguez, Aparo, or Kubert covers.
So add the baseline number of "artist" collectors, to the upsurge in "horror" collectors, to the huge surge in BA collectors over the last 5 years and you have demand for these books coming from several different directions. I tend to believe BA Marvel horror (what there is of it) may be maxed out, but the relative scarcity of DC's to Marvels, combined with the factors delineated above, lead me to believe the popularity of DC horror is still waxing...