People are saying puritans or religion, but the US was even more religious and puritain 150 years ago, when nudity in public bathhouses was common. What changed was the US got rich enough to buy millions of tiny single person bathtubs and make installing more of them than you need an investment asset that you sell to pay for retirement.

Japan has onsens and it also has people sleeping in a bush on the street. Both of those disgust Americans because you look poor. Even if you’re a sleepy business man in a suit it’s offensive that you’re not embarrassed that somebody might think you’re poor for a second. American onsens were for the poor, then for the very poor, then they were effectively illegal because we hate the idea that somebody too poor to buy a personal bathtub is allowed to live.