Every literary genre has its subgenres, but there is perhaps no genre so packed with niches as horror fiction. You've got your supernatural horror, postapocalyptic horror, fantasy horror, sci-fi horror, comedy horror, and then all the vampire, werewolf, and zombie horror. It's a long list of genres for the long list of authors who self-publish in this increasingly fractured and versatile category.
Some horror writers are making a killing at self-publishing, but that's far from the norm. More likely, self-published horror writers are seizing independence to get out work that isn't finding a home with traditional presses-and many of them are passionate enough to keep going despite making little profit.