It could be, but the federal government still needs to be involved in interstate trade, as well as international. Some comes in from Canada, and cocaine is far from the only thing Mexicans are killing each other over when trying to control smuggling in from south of the border. I’m not going to argue that the declared War on Drugs has been successful, just that without the demand for it addressed successfully, it’s never going to be.
I don’t think legalizing it will address that - people already have enough problems with addictions to legal prescription medicine, tobacco, alcohol, caffeine, whatever. Why have so many people chosen to escape reality with narcotics? What is it about them? What is it about reality?
Actually, it's the Drug Warriors that have a good idea why. The principled among them argue that drugs serve as an escape from the onerous duties that the citizenry sometime have to assume. Those include political duties.
The libertarians's riposte is that forcing people to be free only acclimatizes them to being forced. Once people are used to being forced, they can be forced to be unfree much more easily.
You may be interested in this quote from Ayn Rand: "Drug addiction is the confession of an intolerable inner state."