I don't think it's so much the idea of the Apocalypse per se, but that it would happen within out lifetime. There are people want to die in a spectacular manner, and there's a certain poetic flair about death-by-Apocalypse that these people gravitate to, and that certain other ways like-- oh say, sudden cardiac arrest lack. When you die by any other manner (as in like, pretty much how everyone will), sure your family and friends may mourn for a month at best, but life goes on. In the greater scope of things, your death is meaningless. But in an apocalypse, you are part of the very last generation of humans. You
are the terminus of human existence, not some random bloke in its median that got taken out by dysentery.
There's also the appeal of certainty. Humanity can be wiped out without much warning by the innumerable cosmic shit-o-murder that fly around our galaxy; stuff like gamma ray bursts, black holes, America-sized asteroids, and even the inevitable expansion of our very own sun. Also,
these. Scary stuff, and pretty much the foundation of Lovecraftian horror. But with the Apocalypse, you have control over when these happen (or at least the illusion of it).
Or well, maybe that guy who cried Rapture just made a killing swindling Christians and you want in on that goldmine. It could happen.