Honestly, I think the reason zombies have gained such popularity is BECAUSE they're zombies and not living, breathing people. Some people -- usually overprotective and uninformed parents -- get up in arms when you shoot a 3D graphical representation of a human.
Zombies aren't human (or at least aren't human anymore) and so there's relatively few objections. Plus, zombies don't use weapons, which has enabled less experienced FPS gamers to play without as much frustration.
Regardless of the above reasons, I'm sick and tired of "killing" zombies. Let's do something different, dammit!