Because they were from Africa, but new immigrants from Africa came to US earn a pretty decent life quickly than those have been here centuries ago.
Because they're minority, but other minorities such as the Jews and Asians are working hard and earning a decent life here.
Because of less chance to get education, but you have affirmative action that you can get into decent college at a huge discount as far as scores go. Also so many financial support for economically disadvantaged families.
Because of unfair tax system, election system... those seem not the key reason to me too.
I might be missing something else.
I think it's probably more of a culture thing, that favors education, hard working, loyal to family, etc. Without that, the civil rights law or various diversity initiatives can only help that much. The black community needs to address that gradually and it's the only way to fix the "poor neighborhood" for good.
In where I live, the elected democratic officers decided to bridge the poor community with "rich" community by building government-sponsored condos right next to or in the middle of up-middle-class communities,the result is that people move out and the house price drops, does not look like a right fix to me.