Yeah how did mankind survive with it legal the first 2000 years... we have done so much better as a society with it illegal for the past 80 years right? Cancer rates have skyrocketed over the same period... how is that for a study? Like i keep saying fine by me to keep it illegal, just treat tobacco and alcohol the same way.
Because civilized western culture understood and realized that it was detrimental to family, God and country and it's use was discouraged. It didn't have to be illegal because it was understood that only the basest of the base would ever use it....if they could get it. Uncivilized tribes, non-Christian mystery religions and pagan cults were the main users. Gee...just like today!