I'd say for two reasons. First, laws are written by a bunch of old people (at least in the head) that love the stuff. Second, full prohibition does not work anyway.
they mean the heavy criminalization of drugs wasnt about drugs, it was about opressing people. Nixon had a problem with counter culture hippies and blacks. The solution was to impose heavy criminal charges for what they did, smoking pot and also herion in the black communities at the time (so I've read several times, I'm no historian nor expert though).
Like if you wanted to oppress middle class white people you could make chardonnay illegial and jail prople who you send the cops to bust for drinking it
They tried to make it illegal and the results were disastrous, one could argue the same for marijuana but the campaign to keep it illegal was much more successful.
In a U.S. context, it is actually really simple. Racism and the age old practice of othering types of people by associating them with a drug (cocaine = rich and white, crack = poor, black and dangerous). That’s it, the full answer is of course a lot more complicated but in the end it is exactly still this dumb and cruel.
politicians across the political divide spent much of the 20th century using marijuana as a means of dividing America. By painting the drug as a scourge from south of the border to a “jazz drug” to the corruptive intoxicant of choice for beatniks and hippies, marijuana as a drug and the laws that sought to control it played on some of America’s worst tendencies around race, ethnicity, civil disobedience, and otherness.
I actually think examining the rise of crack in the US and how it was used as a political wedge and xenophobic tool of fear mongering helps explain why marijuana is illegal in the US the easiest, because the forces and structures are the same for crack being highly illegal as they are for marijuana, just much less thinly veiled and dialed up to 11.