Well, I personally LOVE the horror genre and would love to do roleplays in that genre. The thing is that the ones I see are just other genres that have an element of horror in them, but aren't really horror. Or sometimes there are combinations with themes that I am horribly bad at (for example: medieval or others in the past).
If it would be pure horror, it would attract me more, but the ones posted are often fantasy, medieval or science fiction with a horror element.