Is it a psychological thing? A lot of scholars claim that horror has enduring appeal because the best horror films reflect unease about our own times, like how Romero depicts racism in Night of the Living Dead. Yet, with films like Paranormal Activity (great movie, don't get me wrong, but it doesn't seem to have a big social message behind it), are horror movies departing from this thematic territory? Or, with the proliferation of horror from all manner of studios, is it just that those "social message" horror films are getting buried?
That's in reference to classic, enduring horror. Works for novels too (Frankenstein, Dracula anyone?). It's not saying that any horror movie with a strong social message will become popular, it's saying that the scariest and most impactful horror films and novels are the ones which are reminiscent of the horrors of the real world