The mafia/smugglers/drug dealers are the only ones that are keeping it from being legalized. Much like the Prohibition, where the mob/bootleggers only rose to fortune and power through illegal alcohol sales, the drug dealers of today will make sure to bribe and threaten anyone that gets in the way of their business.
In Mexico there’s a war going on where thousands have been killed already, only because the trade is illegal in the US.
Additionally, there is always the “mayim gnuvim yimtaku” psychology. When people have strict rules, they are more likely to do the act than before.