Zombies fulfill our need for a "Them". In the past that has been Indians, Krauts, Huns, Nazi's, Nips, Gooks, Slopes, Commies, Terrorists or whatever other term of derision we used for whomever then current "Badguys" are.
Since is is now politically incorrect to single out any group or tribe of people as the "Badguys" without inflaming the sensitives of some mouthpiece .org representing that group, zombies (and aliens to a lesser extent) fill that niche/void of "Badguy" without insulting any group.
I don't see that going away anytime soon.