This is taken from an interview on thewrap.com:
Robert Kirkman reveals why zombies don't actually exist on "Walking Dead"
So, why are the living dead referred to as "walkers" instead of zombies on "The Walking Dead"?
One of the pleasures of watching AMC's new "Walking Dead" aftershow — "Talking Dead" — is the chance for fans to get answers to questions like that one. "Walking Dead" comics creator and TV series producer Robert Kirkman answered it on Sunday.
In the world of "The Walking Dead," he said, the concept of zombies doesn't exist.
"One of the things about this world is that people don't know how to shoot people in the head at first, and they're not familiar with zombies, per se," Kirkman said on "Talking Dead." "This isn't a world the (George) Romero movies exist in, for instance … because we don't want to portray it that way, we felt like having them be saying 'zombie' all the time would harken back to all of the zombie films which we, in the real world, know about.
"So by calling them something different, we're kind of giving a nod to … these people don't understand the situation. They've never seen this in pop culture, this is a completely new thing for them."