The English colonies in North America adopted the Spanish practice of importing captive Africans as slaves. When those colonies later became the USA in 1788, they preserved that system (which they had evolved into "chattel slavery" that recognized black people as if they were tools or livestock). They ceased "importing" kidnapped Africans in 1808, and states began to schedule moratoria on slavery. Slavery was ended after approximately 250 years of practice in 1865, resulting in instant widespread unemployment for four million former slaves.
These newly freed people had no education, little experience of business or finance, and considerable resentment toward their former "owners" - and were concentrated in the southeastern USA (where the white population was only seven million and where the economy was in a shambles following a four-year war and economic blockade). Small and ineffective provisions were made to provide education and to incorporate them into civil and economic life, but these were resisted by large parts of the white population, most of whom had no prior experience with black people in any role but as property. Such efforts were abandoned in 1877, as quid pro quo for settlement of a disputed presidential election.
There ensued a further century (plus) of repression against black people by white people (who saw them as threats, on several levels), attempted migration outside the Southeast by black people to escape white repression, and commensurate spread of anti-black attitudes and legislation across the USA - accompanied by hardening resentment among black people against white people. The disdain for authority (and the tacit non-cooperation with it) that could be expected among slaves toward their "masters" became nearly universal, and an accepted facet of African-American culture. The experience of both slavery and of being suddenly cast adrift en masse, in an inhospitable (often hostile and violent) white society have combined to sharply distinguish black culture from the various European-American cultures. The systematic exclusion from education and employment, and the resulting ignorance, malnutrition, and poverty among the liberated-but-excluded-and-repressed descendants of slaves were not again addressed until the last part of the 20th century, by which time they had left their mark on the distinct African-American culture. There was a corresponding mark left on European-American culture (atop the marks left by the experience of being "masters" during chattel slavery, and the experience of being locally outnumbered by emancipated slaves).
To sum up, by the 1960s, there was a centuries-long history of some European-Americans regarding African-Americans as inferior, dangerous, and as a source of social ills...as well as some African-Americans regarding European-Americans as malicious, scheming, dangerous, and as a source of social ills. Many within both groups hold the conviction that these are inherent "racial" characteristics. This cannot be erased from either culture - it can only be over-written. But neither group is convinced of the wisdom of setting out to deliberately over-write portions of their cultural memories.
That is why there is (and probably will always be) a special, enduring, and tortured relationship between white Americans and black Americans, which is unlike their relationships with any other groups in the USA. The experience of slavery, the instant, mass termination of it, and the repression that followed have indelibly shaped that union, with "racism" as a strong presence among both sides.