Press pause on your remote for a second. This will only take a few minutes.
A while ago, I was moderately good friends with a writer/director of horror/thriller films, and I remember a conversation with him where I asked "why all the horror movies lately?" as it seemed like there had been a surge. He gave me a fairly reasonable answer, one that I felt was actually kind of intuitive after he explained it: it's about escapism. More specifically, the worse the state of the country/world, the more people want to get some release via escaping into a fantasy world - and when there are truly scary things out there, it's more desirable to escape into a world with horrors that far outstrip our own, to make ours seem easier to deal with.
Cut to today where TV is the new cinema. It just makes sense that in the days of ISIS and Ebola, the ranks of The Walking Dead and American Horror Story (5 & 4 seasons in, respectively) have been joined by Sleepy Hollow, Helix, Constantine, The Strain and Z Nation (the last 2 I haven't watched, but I've read the graphic novels and some of the first novel of The Strain). Even shows like Hannibal and Gotham which are not horrors, but are intensely dark and at least in the case of Hannibal far more frightening than their cinematic counterparts. Every time I watch the opening credits of Constantine, I think "damn... I can't believe they get away with this, when some (admittedly crazy) people decry even Harry Potter as satanic."
This is not to say that there are not a glut of offerings in the movies... far from it, but where TV would barely touch the genre (and when they did, it was often relatively light, with a few exceptions), now it is wholeheartedly embracing it, and I still think Dave's postulation holds, it's just taken over the new medium as well. I think this in itself shows how much television has changed. Much has been written about how The Wire, The Sopranos, and Mad Men have "changed the TV world", and it's not for me to say what all this new horror on TV means, but I am more than enjoying my time watching demons, vampires, zombies and sociopathic cannibals step off the big screen and into my living room. Now let's get back to the show.