There are two main schools of thought on this topic, one held mainly by liberals and one by conservatives.
Liberals tend to believe that this is the legacy of past and current racism. They would argue that blacks were oppressed for centuries when they were slaves and then for almost another century of segregation. This caused them to be poorer and less educated. Not enough time has passed and not enough effort has been made to ensure that these effects will have disappeared. This past discrimination is added to whatever amount of racism still exists and cited as the cause of racial inequality.
Conservatives tend to argue that there is a "culture of poverty" among poor African Americans. They have developed (largely because of misguided government programs) a culture that does not value things like education and hard work. Because of these pathological conditions within the lower classes of the black community, a large portion of African Americans remains poor.