It’s simple. A disproportionate share of racists and nativists are in the conservative wing, and more to the point conservatives are ideologically resistant to potentially aggressive government interventions to eliminate discrimination. So racial minorities are forced to ally with white social liberals. But that alliance doesn’t come for free. Social liberals insist on driving the bus on non-civil rights social issues, such as norms around marriage, child bearing, and premarital sex. (To this day, Black Democrats are far more likely to call premarital sex immoral compared to white Democrats. On that and similar issues they’re actually as conservative as Republicans. But that aspect of the culture is muted as a result of the alliance with disproportionately white social liberals over civil rights issues.)