White people push back against black kids going to their schools, businesses, etc. Few good jobs for blacks. Eventually white people move out, bringing their businesses, and revenue, with them to the suburbs. Money doesn't come into the city, no new businesses are developed, racially-divisive restrictions from banks make things worse. Crime and drug abuse increases, partly due to the squalid nature of the resulting segregation and lack of economic stability. Riots in a reaction to all this and the biggest civil rights leader of the century dying verifies white people's worst fears and the white flight moves into overdrive.
With record low access to education, jobs, housing, record high drug use/crime, and racist authority figures/policies, what was a not very good situation becomes a terrible situation, and literally the whole society is engineered to fail, with no recourse for improvement.
Read my other comment below for reading material.