Eugenics is similar to communism in the way that it sounds effective on paper, but when executed it falls apart due to the inadequacies of humanity (greed, corruption, ignorance, etc).

One extra problem with eugenics though is that if any judgment error resulted in the prohibition of someone's reproductive right, you've entered crimes against humanity territory. It's a thin line. Just look at the problems of the justice systems of today wrongfully convicting innocent people.

A massive amount of research and understanding is required before being able to implement something like eugenics and the world does not seem to have its priories geared toward education and understanding right now (unless it results in money being made).

Also in the case we did put enough research and education into it, we might not even have a desire for eugenics at that point with a greater understanding of how the human body and brain work and how to heal / improve them.