...because they encouraged young black women to have kids out of wedlock. Read Thomas Sowell's critiques of the welfare state and the effect that it had on AA families from the 1960s onward. Other ethnic communities, like Japanese, for example, simply have incredibly low rates of out of wedlock births.
But my main point is extramarital sex and alcohol use lead to the breakdown of the family unit, and nowhere has that been more devastating than in the African American minority communities that the author is writing about.