Why? Human weeds are what gives Progressives power. Without the poor underclass to give them votes at an election, the progressives would have no support. They would merely be sitting around in coffeehouses, congratulating themselves about how superior they are to the average working man.
This is precisely why Leftists went from being supporters of eugenics to opponents of eugenics and promoters of dysgenic policies. Had their eugenic scheme been implemented, there wouldn't be a permanent victim class around to demand liberal largesse.
Conservatives who fear eugenics are living in the distant past. They should be more worried about how our current policies are actively dysgenic: i.e. importing the Third World underclass by the millions due to liberal immigration policy, and rewarding the underclasses for having more children through increased per-child welfare checks.