Prison is not rehab. Do you want a massive tax increase? We would need one if we were to actually incarcerate drug users for the duration they need to become sober. And we would need to fund those prisons to a level wherein they don't make people more likely to break the law once they get out. And what about re-entry? It would take a lot of money to not dump former addicts back on the exact same street where they first became an addict. Their friends will be waiting!
Besides, most states still jail people for petty drug crimes and they have been for 70 years. It doesn't work.