The Walking Dead is the top cable series of 2013, as measured by total number of viewers.
Why is horror so popular of late--especially zombie-related horror?
Certainly not because anyone really believes that a mysterious virus is going to result in a worldwide outbreak of the undead anytime soon.
Horror is most effective when it functions as a metaphor.
The zombie apocalypse is a metaphor for the disintegration of society, the downfall of the world as we know it.
Over roughly the last ten years, the Western world has been rocked by uncertainty--starting with 9/11, and continuing with the terrorist incidents in London and Madrid.
Then came the global financial meltdown of 2007-2009.
In the U.S., the economy remains sluggish, and there is not much confidence in either of the country's two major political parties. Europe, likewise, is in political and fiscal disarray.
It is worth noting that horror was much less popular in the 1990s, when the world (at least the Western, English-speaking world) was generally much more stable and prosperous.
Horror fiction is the fiction uncertain times.