There were several things that played into both.

Nazi Germany-Following WW2, Germany was forced into reparations by the allied powers, and all aid was cut off to them immediately following the war. This obviously left the country at a similar low to where it was before the war, as the economy was in shambles. Add to that the Paris Peace agreement, which led to the arrest and trials of Axis war criminals, among other things. Germany was a country in shambles, with its people afraid and humiliated. This alone was enough to have most in the general public denounce loyalty to the Nazi Party.

The dismantling of racism and racial eugenics in America is a much more complex issue, dating back to the civil war. Essentially, predating the civil war, it was normal thought that African Americans were literally a lesser species due to crock science and propaganda from large plantation owners in the south who had huge financial implications in the existence of free labor. Post civil war, up until the civil rights movement. There was still a significant portion of the population who lived through the Civil War, and many in the South called "Lost Causers" believed that the narrative of the Civil War was misconstrued and the South was largely justified in its actions. In short, it took a very long time for this thought to dissipate.

Anti slavery sentiment gained traction in northern states following the Civil War, which served as the base for the depopularizing for race based discrimination, with some policy allowing black people to vote. But to put it short, we still had a LONG way to go. Something called a Jim Crow law was in place that allowed racial segregation in southern states. This is why when you hear someone say the civil war was about states rights, you should be very critical. States rights for what?

The dismantling of popularized racism can also be at least somewhat explained by the Civil Rights Movement and the Franklin Roosevelt/Lyndon Johnson presidencies. FDR, many would say, was the president to bring in what we consider modern progressivism, and his presidency is responsible for many of the equal rights policies we still see today. For instance, he implemented the Fair Employment Practice, which outlawed the hiring of a person based on race, creed, etc.. Jim Crow laws were abolished in 1964. All these things together allowed African Americans to become more prominent voices, entertainers, athletes, etc.. And many in America got on board with the idea that its stupid to see someone as sub human cease they are a different color; but obviously the problem still permeates, especially in the south where generations of racism are still alive.

I could go on and on about both, but civil policy and the humiliation of people on the wrong side of history went a long way in silencing hate based politics. Hope this helps!