Entrenched interests
In addition to people profiting covertly from illegal drugs, there are people who profit from the continued illegality of minor drugs like marijuana.
Case in point: Nevada. You'd think that the state that has legal prostitution and gambling wouldn't bat an eyelash at legalizing pot (to the extent possible under the unConstitutional Federal drug enforcement regime). But you'd be overlooking the state's liquor industry, which mainlines its product into the casino industry. It'll be a cold day in hell before the two industries that own the state government allow legal marijuana to cut into profits.