Just because some countries consider it illegal and are not aware of the medical benefits of it.
I had a neighbor once, they had to move to a different state because his daughter had this medical condition that made her hit herself from time-to-time. Cannabis allowed her to calm down.
People will believe in the dumbest shit as long as everyone around them believes it too. The world is literally filled with insane people.
Fortunately the internet makes it much easier to educate yourself and information spreads much quicker than it used to. In 100 years many drugs that are illegal today will be commonplace around the globe, not just Cannabis.