This question is posed, and not answered, in the podcast in which Loury, McWhorter, Steele, and Steele discuss the latter’s Michael Brown documentary.
Conservatives want to blame the War on Poverty and welfare programs. This story is exemplary normative sociology–the study of what you want the cause of a problem to be. The problem is that the dissolution of black families preceded the War on Poverty.
Loury points out that the black family was stronger in 1930 than in 1960. What happened in the meantime?
My thought is that what happened was the Great Migration of blacks from the rural south to the urban north. One can imagine that this produced a cultural shock that could have weakened marriages through a number of channels.
1. Lowering the status of the black matriarch. Your rejected your grandmother’s rural ways, so she could not apply moral pressure on you to follow marital norms.
2. Greater inequality among black males, weakening marriage. The poorer males are undesirable husbands, and the richer males have leverage to disdain monogamy.
3. Communities no longer church-centric, so that there is less social pressure to follow marital norms.
4. Availability of many more opportunities to have sex outside of marriage.
Trying to tell this migration story leads me to ask why a similar drift toward family breakdown did not occur among Italians, Irish, and others when they migrated in large numbers to the U.S. Perhaps they just had the good fortune to undertake these migrations in an earlier era. People who arrived between 1880 and 1930 had a very hard life, with little time to pursue sex outside of marriage. Also, this was before Freud and others had convinced people of the need to be less repressed about sex.