Only speculating, but maybe it's because the legal status of psilocybin prevents all but the people who are most interested in it from taking it. If it were legally available then maybe more people that would use it casually would emerge than if it were illegal, and the number of negative experiences would increase. From what I understand, it's best to have a good mindset and have someone else knowledgeable of the substance to guide you through the process, otherwise it isn't as effective or harmful to the psyche.
Alternatively, maybe if it were legally available people who took it wouldn't have the stress of "what if the cops show up while we're tripping" hanging over them while they embarked on a drug experience that can magnify that sort of thing into a panic attack, and the rate of negative experiences would be lower.
