I suspect a large part has to do with a selection bias. The people most likely to turn into a bad cokehead are also the people with the most access to it. Most people will never come into contact with cocaine, but they will come into contact with alcohol and likely consume it. The average person is more likely to become an alcoholic, while only certain segments are likely to even have the potential to become a coke addict and I’d bet on those segments being more likely to be harmful.