As a matter of fact, it has actually been linked by several economists to the shrinking Middle Class in our society, and the reduced upward social mobility we have experienced over the course of the last few decades.
Quite frankly, right up until the 1970s, they had all three of the former, and were well on their way towards cinching the latter.
African Americans, for whatever reason, simply happen to have largely "dismantled" their own pre-existing culture and family structure over the course of the last few decades. The Dems bear some blame for that, in supporting policies which made this negative transition easier. The GOP might very well bear some blame as well in their overly-enthusiastic pursuit of the "drug war."
However, the major impetus for that change has always come from the black community itself, and always will. Until the attitudes which make this state of affairs possible reverse themselves on some fundamental level, the problem isn't going to be going away any time soon either.
Any which way you want to look at it, the bottom line here is that if the black community wants to get ahead, they are going to have to do it for themselves. No one can do it for them.