The world of A Canticle For Leibowitz is not our future. It's our distant past.The world of the book is simply too different from ours. There are immortal beings and mutated zombies, and what appear to be psychic powers. When the second nuclear war destroyed the world, the ship of refugees that was sent out to preserve mankind landed on a conveniently habitable planet which was then named after Earth-that-Was. Thanks to Eternal Recurrence, our present civilization ended up taking on more than a few features of theirs, minus the Medieval Stasis. Of course, history has become so muddled over time that our origins have been long since forgotten, but the story was preserved in a chronicle of the events by an obscure author who never wrote anything else. Oh, and Benjamin? He's dead. Atomic blast hit him. Nothing could be done.