I think that this motif has become popular precisely because it appeals to people who know that they cannot possibly justify their incoherent and irrational fears in the real world. Therefore, it becomes appealing to political and religious extremists who are increasingly out of touch with reality. Naturally, religious extremists will not officially endorse the genre, but they can still support it on some level, because they believe in a version of apocalyptic thinking. They simply adopt a different version of the apocalypse that is decidedly more low-tech and bloody.
Likewise, the tea party and their assorted kooky, conspiratorial brethren are so out of touch with the real world that they cannot possibly justify their beliefs in the light of day. But perhaps they can still escape to the fantasy of menace like "zombies" as a surrogate for "communists" or more generally the "liberal bogeyman" par excellence. Their defeat in recent elections, and their more general loss of control in society seems to motivate them to fantasize about a world simple enough for them to understand and where every problem really can be solved just by pulling a trigger. In that world you really do need guns 24/7 bristling from every orifice.
Of course this is not the real world. It's so far from it that, even on a fictional level, this story line needs to be entombed again, perhaps to be resurrected again a few decades later by some other group that feels beseiged by the cruel realities of life.