People are dead, but they're walking. Hence, The Walking Dead. It really is that simple. The Shuffling Zombies or The Moving Corpses don't have the same linguistic punch as The Walking Dead. Two simple words that taste great (but smell awful) together.
The title may be simple, but it's not without irony. For a book called The Walking Dead, the story isn't really about the zombies. Sure, there are zombies all over the place, but the story is more about the interpersonal relationships between the characters, and some of their drama doesn't even have to do with zombies. The zombies just heighten the tension. Because of the despair and hopelessness that's everywhere in the story, you can also imagine the title referring to the living who are wandering around in a state of numb confusion.