I don’t believe there is a single identifiable reason. I would like to suggest several of the reasons.
First is that the civil rights movement made discrimination against blacks a very visible unacceptable behavior. The same is true for discrimination against women and Jews.
I don’t think it is accidental that all of these changes in American behavior occurred at the same time for no reason.
I would suggest the confluence of two historic forces. The first is the fall of our hereditary upper class that occurred about 1960. The second is the emergence of the vast number of hippies.
The fall of the hereditary upper class I believe was a simultaneous wiping out of all class distinctions including the unique classes of blacks, Asians, women and Jews. It is my contention that hereditary upper classes around the world maintain anti-minority prejudices in order to suppress societies' efforts to support and reward merit.
The hippies were the first young group in world history to totally reject Left wing ideology. They rebelled against the entire hypocrisy of their parents and predecessors. One of the most rebelled against attributes of their predecessors was a highly sorted society. The hippies were fanatics about egalitarianism, love and communal sharing. That never fit Left wing pro-proletariat fantasy nor the Left wing love of powerful government.
I suggest that it was the convergence of these two major trends that dissipated the earlier prejudices.