One of the things that I'm curious about with these horror movie tropes that are very popular is how zombies became one of them. I understand where the vampire and werewolf tropes originate from — repressed feelings, especially sexuality — but the zombie thing is another thing entirely. The first two monsters are ancient, but zombies, in their modern incarnation, are very recent.
The original definition, which is hundreds of years old, is a a person controlled or entranced by a wizard to do his/her bidding (and the wikipedia page is well-researched in that regard). The modern idea of zombies didn't appear until the George Romero film Night of the Living Dead as well as the earlier Richard Mattheson novel , which created the idea that zombies were formerly dead people who rise up and search for the flesh of the living. I believe the whole zombie desire for specifically brains concept comes from the Living Dead movie spoof Return of the Living Dead, which turns the whole genre into a rather frightening joke.
But back to zombies: why? I wonder if their general lifeless malevolence, that ceaseless desire to kill the living in the most horrible way imaginable, and the fact that their condition is fairly contagious, makes them a stand-in for whatever anxiety consumes us. Those on different sides of the political aisle often refer to the other side as zombies, giving life to the assumption that people on the other side must be that way because there's some defect in their condition. Thus the zombies become liberals, conservatives, global warming deniers, Macintosh fans, PC fans, etc. That which you fear most, the zombies becomes. (A good discussion of this idea here.)
Of course the zombie apocalypse is a trope that's very popular within the geek/scifi/horror communities. Here is where whatever it is that created zombies, radiation, a virus, gas, Rush Limbaugh (I kid), makes the zombie infestation so strong that society itself becomes overwhelmed. Of all the Apocalypse scenarios, this to me is one of the most terrifying. A nuclear apocalypse is, of course, horrible by definition, but an apocalypse where the living are steadily eaten by the dead, then reborn as a zombie, is a fantasy of gore and death that is truly terrible.
I think what might make zombie apocalypse scenarios compelling is that there's always the possibility of surviving one through cleverness, viciousness, skill with with weapons, luck, or, like Jesse Eisenberg's character in the zombie comedy Zombieland, “cowardice”. There's not much a person can do to survive a nuclear explosion, and, even if you do, what's the point? The zombie uprising tends to lend itself towards geeky fantasies of relying on tools, especially weaponry, and cleverness to survive, hence we have Max Brook's book The Zombie Survival Guide, book of survival tips for that seemingly inevitable rise of the dead.
Zombies also fulfill a cannon fodder fantasy. Anybody who has read a reasonable book on war like All Quiet on the Western Front will come to an understanding that no matter how much the other side in war is demonized, we know that they are normal people like you and me. Every enemy killed is a son or daughter lost, a child growing up without a parent, or a widowed spouse. Zombies seemingly can be eliminated without this problem. While they were people, they're already dead. If these mindless things are not destroyed, then they will never cease their efforts to destroy us. For someone with a penchant for shooting human-sized targets, what could be more satisfying? Your only limitation is how much ammunition you carry.
I wonder what the next monster trend will be? In the meantime, don't bug me. I'm busy playing Plants vs. Zombies.