That's a very interesting question, maybe better phrased as "why would a white-dominated psychiatric profession diagnose more mental problems among underprivileged non-white people than among white people?"
(Also worth noting: there's a strong correlation between cannabis (marijuana) use and schizophrenia. The drug warriors tend to get alarmed that this means cannabis use causes schizophrenia. But schizophrenics also tend to smoke tobacco, drink, and use other drugs. What if they're really self-medicating, and the cannabis correlation is there because smoking a joint makes the voices go away for a bit? I suspect the answer to this question is again down to cultural norms: privileged white folks determine the norms and set the laws, and this works against underprivileged/minority folks. Oh, and privileged white folks get to pay lawyers to either get them off drugs charges, or get them a lighter sentence. Hence their under-representation in prisons. Full disclosure: I am a privileged white person.)
(Also worth noting: there's a strong correlation between cannabis (marijuana) use and schizophrenia. The drug warriors tend to get alarmed that this means cannabis use causes schizophrenia. But schizophrenics also tend to smoke tobacco, drink, and use other drugs. What if they're really self-medicating, and the cannabis correlation is there because smoking a joint makes the voices go away for a bit? I suspect the answer to this question is again down to cultural norms: privileged white folks determine the norms and set the laws, and this works against underprivileged/minority folks. Oh, and privileged white folks get to pay lawyers to either get them off drugs charges, or get them a lighter sentence. Hence their under-representation in prisons. Full disclosure: I am a privileged white person.)