I've been asked the question a couple of times, and it is a valid one. Clearly, if I wanted to get rich from writing, I should be focusing on paranormal romance (it turns out this is now an actual genre! I had no idea) or erotica (the sheer amount of this stuff being self-published is mind-boggling, but apparently the market is there for it. Again: no idea).
So why zombies? The market for new zombie fiction is probably near saturation point, and any new zombie tales are inevitably going to look like Walking Dead cash-ins.
The simple answer is that zombies are awesome. I could just leave it there, but there is a less simple answer. I always felt like I had to start with zombies; like I couldn't focus on any other type of horror until I'd removed zombies from my system. It's strange, the allure of the undead. I felt it - as many must have - when I was pretty young, watching Romero's Dawn of the Dead, and finding myself thinking not 'this is horrific' but 'this is amazing! I wish I had my own shopping mall ringed by a moat of walking corpses'.
It's a little odd I guess, but I know I'm not alone in secretly pining for the zombie apocalypse. I hear it come up in conversation, I see it reflected in the success of things like World War Z (book and film). There is something fundamentally cool about the zombie apocalypse, something magnetic.
I think it might be something to do with the times we live in, the feeling that our lives are shaped by, or chained by, circumstance, and that the world is fundamentally unfair and there's not a damn thing we can do about it. We're teased by the lives of the rich, or notions of success that will probably never happen for all but a lucky few. The zombie plague removes all the restraints placed on us by social status or wealth or employment. It offers a blank slate, the chance to have a stab at a genuinely meritocratic life. Zombies don't discriminate: they'll eat the rich and powerful too. It is the etch-a-sketch apocalypse, a chance to shake the world away and start over from scratch.
Or maybe that's all just reading too much into things. Zombies, after all, are awesome.