I think the main reason Eugenics never takes off is because the application of it is, almost universally and by necessity, monstrous. In theory you can look at it and say: "Well, if we just ask people with genetic markers that suggest their child would have a serious disability to adopt instead of breed, we'll be fine"... But what do you do to enforce that law? If someone has a child without your permission, do you kill it? Do you kill babies and then claim to be the good guy? Do you make the parents impotent and barren? Do you chemically force a person to be unable to breed with anyone else under any circumstances and then give yourself a gold medal? Do you just lock the parents up, and if you do, do you just toss the kid into the foster care system? Do you effectively orphan a child and ruin it's life in the name of not creating children who will have ruined lives?

If you don't do one of these things, do you have a law in place at all? There's no repercussions and nothing stopping people from disobeying. You basically have a very strong suggestion.

So in order to perform Eugenics you, essentially, have to start by agreeing to be a monster, even if you're doing it with all the best intentions, and once you're considered a monster, people will want to take away your authority to continue doing what you're doing, and now you have to be an even greater monster if you want to continue by doing it against the will of the people. That means tyranny.

There aren't a lot of people who will sign up to be hated and reviled and almost inevitably removed from their position unless they are willing to try and become a tyrant who rules through violence knowing that it's liable to end poorly for them even if they had the absolute best of intentions... And then there's also the aftermath:

Okay, so you're in power and you're passing down laws to decide who can and cannot breed. You're doing it because you love humanity and you want it to have a bright future free of certain diseases, mental illnesses and deficiencies of thought that you've identified are passed down genetically. You're the Knight Of Infinite Resolve and you're doing your thing... But when you retire, or are killed, or just leave power some other way, what about the next guy? With this law of breeding in place, what if he just decides: "Well hell, I just really hate the Koreans and their KPop bullshit, and also people with that frizzy hair crap. Those guys can't have babies now."?

In theory it seems straight forward, but in practice it's a minefield and it never ends well. It's like Communism. If I said to you: "Let's all just share everything. That way if the country is doing well, we're all doing well, but when we all need to pitch in, there aren't people who are still doing great and others who are suffering!"... But then you end up with a Citizenship Score and your husband is the in the gulag for thinking out loud and the mayor is living in a mansion with a different car for every day of the week while you get rationed a single turnip per working family member and you come to understand that things which seem good in theory can be much more difficult to action once 7.5 billion other people all get involved by being alive and having their own wants and desires... Damn them!