It's an interesting question. I imagine a lot of it has to do with our current world view. I think a lot of people have a tacit understanding of how fragile it is.

The psychedelics for example aren't illegal because of any harmful physical effects - they're some of the safest drugs we know of, physically. They're illegal because of the world dissolving experience you might have on one. For a people like us who have built up an elaborately abstract way of viewing our world, this is quite threatening.

I think it can also speak about why something like marijuana is still illegal yet caffeine is a daily sacrament for us - one makes you go even faster and work even more, the other makes you fairly uninterested in such things.