I believe that the reason marijuana is not legal is that it heals possibly even more than it harms. And I believe that it is Big Medical that wants it to remain outside of the people so that they can make money off their drugs, and off sick people.
Consider. You can shop around for a doctor who will prescribe almost any medical drug that you want. On the bottle it says something like, "take one a day." But if you don't listen to what the bottle says, and you take 5 a day, depending on the drug, you might get a bigger high than you could ever get from marijuana. You are the one who controls how much you take. And you can get more of the drug over the border in Mexico... or elsewhere... even if the doctor won't prescribe more.
The point is, you can get high on marijuana or on a legal drug. You can wreck your health on marijuana or on a legal drug. But if you misuse marijuana or the legal drug, it is still you who has the control to do so. All you have to do is shop around for the legal drug - which makes money for the medical - or you can grow your own marijuana - which is essentially free (if you can get away with it).
Big Medical doesn't want you to grow your own marijuana. They don't make any money off you that way. And worse, marijuana used in a right way cures a persons pain. In addition, it has been found that THC cures Alzheimer's. And CBD oil cures all kinds of other maladies. When you are cured by something that is free, Big Medical can't make money off you.
It's all about money, and who gets your money.