I grew up in a cult, so my perspective is a bit off. Did people actually reject eugenics after WW2 or did they just start whispering their approval of it?
The nazis were very upfront about how they got their thoughts on eugenics from America. The way we treated Black people and the native population. Especialy the nonconsensual sterilization.
When the nazis realized they were losing, they switched to killing everyone instead.
It's kind of hard to watch that happen and the majority of the world unite against it then saying "yeah, but we'll just go right up to that line".
To a lot of Americans it was a wakeup call. But what changed American culture the most was minority service members coming back after being treated not just as equals, but heroes by Europeans. A 1940s Black man from the South would have had their minds blown at even being treated like an equal.