Because outlawing stuff shifts incentives to favour the most violent entrepeneurs?
Outlawing does not decrease demand, making stuff real expensive, so everyone wants to sell or buy it, but there's no legal protection against fraud or theft, so to meet demand, violence must escalate.
Just look back at prohibition, the drug war, outlawing guns in Britain eskalating knife violence, etc.
The way to handle the problem is not by outlawing products,but by reducing demand for them.
Why do people want drugs and alcohol? Generally to self-medicate. Mitigate the underlying causes,and watch demand evaporate.