To bang a fairly weathered drum, this is largely a matter of conservative policy. Psychedelics were far more common in leftist circles (some might argue that this is cause as opposed to effect - mind-expanding experiences tend to shift politics towards that end of the spectrum).

This means that if you wanted to use the law as a political cudgel without being accused of thought-policing, you could outlaw psychedelics and be confident that the blast zone would exclude the politically faithful while locking up loads of political undesirables; like hippies, panthers, etc.

Of course, these days you have right-wing podcasters discussing DMT with billionaire CEOs, so it's a little trickier. Thus the tentative steps toward legalization.