“why”
Because it’s a consciousness altering drug. To handle a gun you have to have your wits about you.
And so does alcohol, would you prohibit people who indulge weapons permits?
Just because people may have a medical certificate, does not mean they spend their lives smoking pot.
Many people use medical pot for medical reasons, that does not mean they are high every minute every day.
It is the same with alcohol, people drink socially or to get drunk or because it eases pain, do you believe they should be deprived from owning a weapon IR concealed weapon?
It's a real toss up as to who I would be more nervous about having a gun: This cancer patient who uses pot that has had the THC (the "stone" component) removed, or a short-tempered dude who just drank a six-pack of beer and several shots of whiskey.
One is allowed to carry, the other isn't.
“Because it’s a consciousness altering drug.”
So is alcohol and many prescription drugs, not to mention simple OTCs as well.