Lots of reasons, some borne in discrimination, some not. For example, white drug users are more likely to deal/live in the middle of nowhere (i.e. the "poor white trash" stereotype), with little law enforcement presence, whereas blacks are far more likely to live in cities, with a much heavier crime rate/police presence.
They do not get caught?
brothers like their ho's,hip hop and drugs
maybe white people are better at hiding it and better at evading the cops?
Juries - which are not predominantly black by those same statistics. So black dd/u are not actually convicted by a "jury of your peers."
Of course they have more money to pay off the judicial system!!!!!!!!!!!!!!!! The latter are defended py public defenders and I'm not putting PD down but its a little harder for judges to take them seriously when given cases at the last second and so on..........
I'm not sure about your statistics. "I'VE HEARD" you can't score good drugs from anyone but latinos and blacks, with 5% being the white number.
Cause they take and sell it in the street? well that's the nicest reason I could find.