Illegal drug dealing is hardest to eliminate when you have eliminated legal drug dealing. People like drugs. Making something legal or illegal does not change that basic fact which has been with us since prehistory.
And, like a lot of things driven into grey and black market territory due to political maneuvering, illicit drug trades are more difficult to regulate or disrupt through legal channels because forcing them to use different channels resulted in a robust and successful network separate from the 'conventional' (aboveground) ones.
