The government spends a large portion of its money on law enforcement. If for example, marijuana became legal, that would be billions of dollars that the FBI, DEA, and Border Patrol would not get. And if they don't get that money, they can't pay their people. If they can't pay their people, their people will start pointing fingers. The politicians of this country do not want that. That's money out of their pocket when it comes time for elections. No politician wants to be the guy that "took money" out of law enforcement pockets. Especially if "crime" is high in that politicians area.
Also, the Prison Industrial Complex is a business. That business cannot survive without prisoners. Many of which are in in prison because of selling drugs. If there are less drug offenses, then there are less prisoners. If there are less prisoners, there is less money to be made because all that money that would go to medical, prison staffing, construction, etc. would not go to the prisons. And again, those businesses when it comes election time will remember the politicians who caused their revenues/profits to go down.
So to summarize, the government has a vested interest in making sure that drugs are illegal.
It's so demonic, friend