To answer your question: Because alcohol has
always been legal. I'd go so far as to say that Western society has been shaped by the availibility of alcohol. Our brand of capitalism (including the 40 hour week, etc.) wouldn't go down so well without any psychoactive drugs, so alcohol fills that role.
I don't think that prohibition is the way to go. "Denormalizing" alcohol, as has been done with smoking, would work out better.