No wonder it has failed. Prohibition has various other effects. It recruits a sales force of small time users who sell drugs in order to help pay for their own consumption. It boosts crime, since those who have acquired expensive addictions will steal to get the money they need. Dealers dilute their products in order to sell them for more and this increases the health risks run by drug users.
Prohibition also leads to police corruption.
Dealers bribe the police to ignore their activities and occasionally give them the names of other dealers, business rivals, so that the police have arrests to their credit and the dealers have reduced competition to contend with. When a dealer is arrested any cash handy and most of their drug supply can disappear. The dealer is not going to complain since a serious charge of possession with intent to supply has been reduced to one of simple possession and for their part the police now have cash and drugs for their own use or for paying informants. The only point at issue here is the extent to which such corruption occurs. Drug dealers are the heroes of the curiously-named free market.