Unfortunately, all good things must come to an end, and when "downwinders" started complaining of ill effects from all that pesky atomic fallout, it marked the beginning of the pooping of Sin City's atomic party. Nuclear testing was forced underground in 1963, and eventually the site ceased testing altogether.
No word on whether Vegas' atomic craze just sort of quietly petered out or if they sent the era off with one final bang, but we're going to assume that they celebrated in much the same way military officials celebrated the end of the "successful" atomic pummeling of Bikini Atoll during Operation Crossroads back in 1946 -- with delicious mushroom-cloud-shaped cake. Never let it be said that The Man doesn't know how to celebrate mankind's ability to wreak horrifyingly godlike destruction with class.
It was iced with vanilla butter cream and filled with undiluted fear.
But at least they were just watching weapons of war being tested. It's not like people used to sit back and watch actual battles take place ...