One of Nixon's top guys explicitly admitted that the motivation was racism: https://edition.cnn.com/2016/03/23/politics/john-ehrlichman-...

    "You understand what I'm saying? We knew we couldn't make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin. And then criminalizing both heavily, we could disrupt those communities," Ehrlichman said. "We could arrest their leaders. raid their homes, break up their meetings, and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did."