'Cause it's the stuff I like to read, basically. I'm occasionally tempted to write romance or attempt disaster nonfiction, but mostly I end up writing urban fantasy and horror because it's the exact sort of thing I look for when I hit Borders. That's how most of my story ideas come around, ngl. If I ever sit there saying, "You know what I'd really like to read?" I inevitably end up trying to write it with wizards and monsters and vampires. :)