Well, on average, black culture places less of an emphasis on scholastic achievement. Also, with a higher single parent household rate, that makes it harder for the children to receive support from parents with things like studying and homework. Culturally, births out of wedlock are more accepted among black people. Some subsets of whites have this issue as well, but it's far more common among the black population.
That being said, there are subsets of blacks that are high achieving and have few single parent households. West Indian blacks tend to perform far better than the average black person here because of a greater emphasis on education. Thomas Sowell discusses this in many of his books and columns.