Because it scratches an itch that other types of fiction can't reach... Maybe not everyone feels that itch or has it keep them awake at night, but I think a sizable group of people do and always have done.
Dark fiction has always been, for me, a way of exploring various emotional and philosophical ideas - I know that sounds pretentious and wanky, but I stand by it. How do we know what's real? How do we define ourselves against the 'other'? Why does the dead and buried past have such a hold over us? If such questions run through your head when you're trying to sleep, you're feeling the itch. And far be it for me to tell you what to read, but I don't think Dan Brown is going to cut it somehow.