The question is this, forget race.
What happened in 1960 that caused the marriage rates of poorer americans to plummet. Whatever the answer, it effected black families disproportionately because more of their population was poor (probably due to racism/oppression/lack of opportunity). The poverty and oppression did not cause marriage rates to plummet, because those things were greater in the previous 70 years when marriage rates were higher. But something happened that was a catalyst. When added to the poverty it created an effect of a greater likelihood of single parenthood in poor people.
So it must have been something else that when added with poverty had a great effect on marriage rates. This is a societal question, not a racial one.
What happened in or around 1960 that made marriage rates free fall? Thats the real question.
Here's a University of Wisconsin paper written by an assistant director of the Equal Opportunity division of the Rockefeller Foundation, that attempts to answer this question. The author posits that it's directly correlated towards the disproportionate urbanization of the black population and the disproportionate amounts of marriagable men versus that of women, in the black community.