Because when Portugal decriminalized every drug, the country saw an increase in drug use that was the same as the increase in other countries that didn't decriminalize [1]
Oh look, it's the gateway-drug argument [0] all over again.
The short version is that, for those other drugs that the grandparent listed in their comment, they are already scheduled for medical usage. Only the psychedelics are scheduled away from any legal usage.
[1] https://academic.oup.com/bjc/article-abstract/50/6/999/40402...