Rampant incest, plain and simple. No one wants to acknowledge it but that's the reason. Sons and daughters born to single mothers who don't know who their father is perpetuate the cycle of extramarital sex and fatherless pregnancies. Compound this with the fact that most Black neighborhoods (hoods) don't have a lot of movement--that is, most of the single mothers don't leave where they settled down and don't venture far for sexual partners. That means in any given neighborhood you have women having children by the same men, and those children grow up not knowing they are half-siblings (because their mothers don't know their fathers and if they did they didn't know who else their father knocked-up).
Set this at a rate of over 70% the entire Black population of the USA (single mother households) and perpetuate it for literally three or four generations. What would the IQ be of someone born from an offspring of half-siblings, born from half-siblings, born from half-siblings, etc?
There's more than one reason why the nuclear family is integral and this is one of them.