The way we teach evolution (at least in the US, but I don鈥檛 think the rest of the english-speaking world does much better) contributes somewhat to this. We lay out a very compelling story about how life evolved, eventually leading to people and every other living thing around us. And what does that mean for our future? Or for you and me today? We don鈥檛 talk about that. So dumb, discredited eugenics becomes this forbidden thing, they don鈥檛 want you to know about.
Now, I think human evolution is really a story of learned behavior. Other species take generations before, by chance, a behavior becomes encoded instinct. And it takes even more generations before that becomes species-wide. People, meanwhile, learn and share new behaviors between one another in a fraction of a lifetime. If the history of all hitherto existing society is the history of class struggles, then it is from the intersection of learned behavior and our material environment that those classes emerge.