The short answer is, don't be shocked: racism.

The longer answer is incredibly well spelled out by /u/hillsonghoods

Here talking about post war prohibition/illegalization of drugs

and here specifically about marijuana and racism.

And if you want to take a further dive, /u/UndercoverDoll49 will tell you here about the racist underpinnings to Brazil also prohibiting marijuana

and if you just can't get enough, you can read from /u/Hghwytohell about more connections between racism and prohibition of narcotics here