Adding to your list: divorce...sexual promiscuity and resulting pregnancies...expansion of welfare programs...encouragement of the victim mentality...generalized decay of the culture
I worked as a substitute teacher in the Houston ISD for a while in the 1970’s and again in the 1980’s. A lot of the kids in some of the “inner city” schools where I was worked hard to get to school, get a diploma and go on to college. There were others whose only interest was to disrupt the class and keep others from learning. I’m assuming that some of those kids who are now adults are among those who find that the middle class eludes them.
Bingo. Divorce, cultural changes and pressures, drugs and expansion of welfare programs.
Also, add the rapid decline of the inner cities from the 1960’s.
It is true that as the American culture has coarsened and degraded, the African-American subculture has done so at an even more accelerated rate.
Anecdotally, when in the temporary labor business I saw many poor hard working black men treated like crap by their women, who then treated their delinquent sons like little Princes of Nubia. It was heartbreaking, to say the least, but scientifically worthless.