Because marijuana is a political drug, just like aids is a political disease.
No doubt the “medical” part of medical marijuana is abused by some, but if it produces relief in those suffering the effects of chemo and cancer, what kind of person would withhold it?
Oxycontin is abused by people all the time as well. I don’t hear a lot of calls to ban it. All things considered I would guess Oxycontin could probably be worse for you than marijuana.