It's because illegal substances have a negative connotation of them. It's ironic because they are not only physically safer than legal substances, but they are also both cheaper and have higher recreational value than them.

Also, if the masses started doing psychedelics, it would be the downfall of capitalism as everyone begins to realize that the world's governments and corporations are full of shit. Psychedelics really are the metaphorical third eye opener they are touted to be.