John Joseph Adams on Apocalyptic Science Fiction

Famine. Death. War. Pestilence. These are said to be the harbingers of the biblical apocalypse—Armageddon, The End of The World. In science fiction, the end of the world is usually triggered by other (or at least more specific) means: nuclear warfare (or disaster), biological warfare (or disaster), ecological/geological disaster, or cosmological disaster. But in the wake of any great cataclysm, there are survivors—and post-apocalyptic SF speculates what life would be like for them.
The first significant post-apocalyptic work is Mary Shelley's The Last ManThe Encyclopedia of Science Fiction, edited by John Clute and Peter Nicholls. (1826), which concerns the survivors of a plague that is wiping out the human race. The sub-genre rose to prominence during the 1950s and reached the height of its popularity during Cold War, when the threat of nuclear annihilation was very real. Though the enemies have changed since the dissolution of the Soviet Union, life today holds as many threats and so post-apocalyptic tales remain timely and potent. For further reading about the history of the sub-genre, check out the "Holocaust and After" section of
The appeal of post-apocalyptic SF is obvious: it fulfills our taste for adventure, the thrill of discovery, the desire for a new frontier. It also allows us to start over from scratch, to wipe the slate clean and see what the world may have been like if we had known then what we know now.
But perhaps the sub-genre is best summed up by this quote from "The Manhattan Phone Book (Abridged)" by John Varley:

"We all love after-the-bomb stories. If we didn't, why would there be so many of them? There's something attractive about all those people being gone, about wandering in a depopulated world, scrounging cans of Campbell's pork and beans, defending one's family from marauders. Sure it's horrible, sure we weep for all those dead people. But some secret part of us thinks it would be good to survive, to start over.

"Secretly, we know we'll survive. All those other folks will die. That's what after-the-bomb stories are all about."

 Taken from:  http://www.irosf.com/q/zine/article/10013