Great Question @lilikoi ! Prohibition proved that you can’t force your religious and social values on others without a lot of unintended consequences like organized crime and bootleggers.
I was around when marijuana became popular in the 60’s and 70’s. It mostly came from you local neighborhood hipiie who grew enough to maintain is supply. Nothing really serious happened if you were caught (as long as you weren’t in the military).
I noticed when the police started raiding growing patches, prices went up and organized crime started to take over. The more the government fought these syndicates, the bigger and more violent they became.
Pressure became so great that foreign growing and importing became more important and better organized. The government reacted by increasing officers and funding, which made international trade even better paying and more violent. When I grew up, nobody was killed for marijuana now it is common.
You ask why it is banned; the primary reason that too many people make their living off it. Very few are the crimminals, the majority are government and legal professionals. There is just too much money to be made fighting a drug that generally makes people non-violent. The other, rather minor reason is that the religious right thinks we should all suffer in this life and any drug that prevents it (at low cost) must be immoral.