Because they misuse their own capacities. So you should sway other to help them better use it. Carrot in front of the donkey.
“I could show fight on natural selection having done and doing more for the progress of civilization than you seem inclined to admit. Remember what risk the nations of Europe ran, not so many centuries ago of being overwhelmed by the Turks, and how ridiculous such an idea now is! The more civilised so-called Caucasian races have beaten the Turkish hollow in the struggle for existence. Looking to the world at no very distant date, what an endless number of the lower races will have been eliminated by the higher civilized races throughout the world.”
― Charles Darwin
"A prevention of the faculty and opportunity to procreate on the part of the physically degenerate and mentally sick, over the period of only six hundred years, would not only free humanity from an immeasurable misfortune, but would lead to a recovery which today seems scarcely conceivable."
― Adolf Hitler
"The way of nature has always been to slay the hindmost, and there is still no other way, unless we can prevent those who would become the hindmost being born. It is in the sterilization of failures, and not in the selection of successes for breeding, that the possibility of an improvement of the human stock lies."
― H. G. Wells
"It is better for all the world, if instead of waiting to execute degenerate offspring for crime, or to let them starve for their imbecility, society can prevent those who are manifestly unfit from continuing their kind....Three generations of imbeciles are enough."
― U.S. Supreme Court Justice Oliver Wendell Holmes Jr., Buck v. Bell, 1927