The same arguments that weee used for prohibition.
What we are doing clearly isn't working. The collateral damage to innocents is tremendous. The cost of the men with guns forcing your moral values on others is astronomical. The cost of all the lawyers and court personnel it takes to prosecute and defend drug charges it incalculable. The cost of imprisoning people who don't agree with your moral values is staggering. You have given El Chapo a monopoly on a product because you think it is immoral.
I wouldn't regulate it at all. I know you can't help yourself because you think getting high is immoral. Regardless of the extent you would regulate the behavior of others, it will be less than it is now.
For what it's worth, I think drug use is about as stupid of a thing a person could do, other than watch The Apprentis. I just have no desire to force at gunpoint what you do to your body. Otherwise I would arrest everybody who watches Duck Dynasty. I think reality shows kill brain cells.