There are two reasons marijuana is still illegal:

1) The cops in the US are largely two types of people: straight-laced rule-followers and abusive bullies. The rule-followers support the rule because it’s the rule, and the bullies support the rule because it lets them continue to beat up theatre kids after high school.

2) Marijuana abuse leads to economic harm for capitalists. People can stay economically productive through tobacco or alcohol abuse but marijuana abuse tends to make people lethargic and low-output. While tobacco harms the individual tremendously and alcohol harms society tremendously, marijuana use, while on an absolute basis nowhere near as destructive as alcohol, hurts the tycoons who call the shots.