Why are there so many vampire films? Or buddy cop films. Or songs about rainbows?
Really, I agree with most of the posts above. There is a school of thought that horror movies should be about the protagonist, rather than the monster. Clive Barker has often talked about this. Odd given that he created some of the most iconic film monsters in the last 30 years (Pinhead definitely sits in the same class with Jason and Freddy. Candyman less so, though the story is REALLY good), but if you look at the first Hellraiser, the focus is never on the cenobites. They aren't even named. Frank and Julia are the real antagonists; the cenobites are a force of nature. A storm during which the story takes place. Exactly like a good zombie film. Again, just one school of thought.
Also, to expand upon SiCurious' line of thought, zombie films are very closely related to post-apocalyptic films. As long as we are afraid of the world (or society ending), there will be zombie films. Doubly so because in a zombie film, the end is Our Fault. Zombies are recognizably human, and a fate that awaits everyone in those films (not everyone is turned into a werewolf, etc, etc). Zombies are dead people, and all people die.
It's not a subtle metaphor (rotting flesh rarely is), but it's a powerful and visceral one, pun intended. That's why a good zombie film rarely explains where the zombies came from. To use the above Dawn of the Dead example, Q:"why did the world fall apart?" A:"rampant consumerism" Asking where that came from COULD be interesting, but usually out of the scope of most zombie films (and can completely derail the metaphor if not done right).