After the failure of the "victories" of the civil rights movement. What was promised in the 1954-1965 period has not occurred, in many ways, quite the opposite has happened.
One psychologically appealing explanation is white racism.
There are others.
Definitely trending in the wrong direction for blacks; a sharecropper could scrimp and save to put his children through school so they could enter the middle class, and that mobility is basically gone now for most of them - illiterate, no work ethic, high chemical dependency...