The first salvo of the attack was with the "Marihuana Tax act of 1937"
That was a attempt to price it out of the market,couple that with a Federal Drug Agency that needed a new "bad guy" to stem the hemorraging of it's budgets and importance,Resulted in an out right ban.
My bet is that "lots of Campaign Contributions" and Scare tactics concerning a certain minority group was smoking it and raping white girls helped push it through Congress