How does the Place of the Gods become a dead place in "By the Waters of Bablyon"?

The Dead Place is New York after some type of cataclysmic event.  Maybe a bombing, since this was written close to WWII.

The young would-be priest gazes over the ruins—with their broken bridges and tumbling towers—and envisions the city at the moment that it died: huge, restless, destroyed by fire from the skies from weapons of unimagined horror, followed by a poisonous mist that left the ground burning for aeons. When he sees a “dead god” sitting by a window looking out on the ruined city, he realizes that the “god” is only a man and that despite its wonders, this city, New York, was once a city of men like himself. He longs for the knowledge they possessed and is sure of his ability to use that knowledge more wisely than they. As a new priest he will help his people make a new beginning, recapturing lost knowledge from the broken city in order to build again.