For everyone else: independent scientific research from both UK and the Netherlands classifies lsd and psilocybin as the least harmful drugs. Look it up.
To answer your question directly, its fear.
People fear what they don’t know.
Governments fear the potential these drugs have to make people feel aware of things they were initially not.
The hippy movement of the 60s was a long term unintended consequence of cia experiment with lsd on student volunteers. They did not like it.