Las Vegas, Nevada was, aside from the possible rivalry of Monte Carlo, the capitol of gambling in the world. High rollers from across the world would come to spend their money and sample the foods, entertainment and vices particular to that glittering artificial oasis.
However... The end came with the advent of the Promethean Nanites. Unlike other cities where the victims were 'merely' mutated into nonhuman forms, in Vegas something went even more horribly wrong. Instead of converting people into new forms, the nanites simply tore every bit of biological matter apart and produced a quivering mass of grey dust coating the vacant city. Few escaped the horrid scene, and fewer still wish to return to the site.
This effect is known as the Grey Goo scenario. One of the nightmare-future ideas semi-predicted by science fiction authors, in Vegas it came to be a nightmare come true.