Everyone Has Probably Worked This Out Already

The reason we've seen such a surge in zombie stories over the past decade is because they fill the role that major-terrorist-attack stories used to: exciting stories set against the backdrop of a huge, apocalyptic upheaval in normal life, which the audience knows can never, ever actually happen. SFnal apocalypses, like Cloverfield, serve an identical function -- the vector of destruction has to be something the audience knows to be impossible.

After 9/11, Die Hard and its ilk were no longer unthinkable, so the big thriller disappeared, briefly, for retooling. When it came back, zombies were the new terrorists -- suitable for the background of all kinds of thriller stories, and then (for the current, second wave) for stories commenting on thrillers, from comedies to romances to buddy movies. Terrorists won't come back in big dumb stories until we're convinced that they won't strike in real life.