Because in the late 60s and 70s, US politicians needed scapegoats to blame for the social unrest, and the psychedelic movement with prominent figureheads like T

Because in the late 60s and 70s, US politicians needed scapegoats to blame for the social unrest, and the psychedelic movement with prominent figureheads like Timothy Leary was an easy target. Demonizing and criminalizing their drugs of choice gave them cause to imprison them and break up their social movements. So campaigns and institutions were established to proselytize the dangers of whatever drugs they were using, and laws were written and regulations established which simultaneously criminalized those drugs. Because the US political system is very slow moving and hard to course-correct, those institutions, regulations, and propaganda continue to this day.