Do you have any insights as to why they might have been made illegal in the first place?
Because telling other people how they must live is an irresistable urge among the small-minded. All of the reasons boil down to that. Drugs are hardly unique in this sense.
If the intention were to reduce harm as much as possible, prohibition is one of the least effective methods and all of the research shows this. But these are not people who are interested in facts, in measuring the effectiveness of their own solutions and no longer using methods that don't work. That would lead to conclusions that would interfere with pontificating to others about how they should live.
"Live and let live" and the notion of "consenting adults" do not occur to the small-minded.