In the 30's marijuana was considered a drug used mainly by Blacks and Hispanics. But when white kids started using it...
https://www.britannica.com/story/why-is-marijuana-illegal-in-the-us#:~:text=Aided by an eager news,illegal across the United States.
Further, pot became a useful tool in arresting anti-war protesters. You can't bust hippies for free speech activities, but you can bust them for weed.