my understanding is that cannabis is far less damaging to the health than nicotine cigarettes. yet nicotine cigarettes stayed legal while cannabis was declared illegal. the logical conclusion is that the health concerns were not the determining factor. therefore, it was obviously about other things, such as cultural perception, commercial lobbying, political ideology.

by the way, tobacco (nicotine) was one of the first cash crops that were exported from the US to europe when america was first settled by european settlers, which gave money to america and made the migration much more attractive to many people. that's why there is a cultural fond memory of tobacco, because it funded settling america back then (among other factors) (source: read it a while ago on wikipedia).

then there's the strong tobacco lobbying group. turns out that there's a lot of money to be earned with cigarettes and the lobbying group for tobacco is strong. that's another reason why it wasn't banned.

then there is this whole issue that a whole lot of drugs are primarily taken by marginalized groups. why? these groups already have a difficult life, drugs often help with pain relief -> higher drug usage in the black population. combine this with the "forced labor is illegal except as a punishment for crime" thing and suddenly you get a rural south that wants to find any pretense to criminalize black people. and guess what, drugs are a convenient way to do that.