This answer is based entirely on hearsay, but well...
So in the early twentieth century, the US put forth an increased effort to regulate drugs and outlaw others, including opiates, marijuana, etc. A wide variety of social factors contributed, but in the case of marijuana, federal lobbies acting on behalf of the cotton industry had a decisive effect. Hemp fiber, which comes from any plant in the cannabis family, is the strongest natural fiber. Also, marijuana isn't called "weed" for no reason: it grows very easily, in almost any climate. In the early twentieth century, therefore, the US had a bunch of slaveless cotton farmers who had been hegemonic due to slave labor but who now faced a competitive threat from people wanting to farm cannabis...which makes stronger and higher-quailty paper, clothing, etc. Luckily, big investors in the cotton industry, including Dupont, had the strength to convince Congress that since certain strains of cannabis (e.g., indica, sativa) qualified as mind-altering substances, well, they should be outlawed along with heroin, etc. And there you go. Big business in the US outlaws a substance way less socially destructive than alcohol. Why? Because of the interests of capital. While capital also spends lots of time keeping bullshit like tobacco...which is much, much more addictive and deadly...legal legal legal.