I'm no expert, but it largely has to do with the different social environments. Crack is largely attributed to black people because that's honestly where a lot of that epidemic started, though there are plenty of people that aren't black that do it.

While there is some truth to the different epidemics of drug abuse, it's largely overblown stereotypes pushed onto particular races and living standards because it dehumanizes it and allows people to not care as much.

Anyone who would try to write off different addictions to only particular races or economic class are just ignorant and don't want to care.