Cannabis and mushrooms, to name some, have been considered and succesfully used as versatile medicine for thousands of years, only to be banned about half a decade ago. Only the last few years this is starting to change, as more and more research shows the medicinal benefits, while debunking most of the lies that have been spread.
Psychedelics 'cure' consumerism, make one more sovereign, enable independent thought, and help you break destructive patterns, by making one see through their life-long conditioning. They make us better people, but worse consumers who need less medicine.
It's quite obvious why these substances are banned, as it's not in the interest of the state to have a flock capable of independent thought, that does not consume a lot and doesn't buy words of division and fear. To clarify even more: the only legal substances are those that cloud our thoughts and make us stupid and dependent, like alcohol or prescribed anti-depressants.