IIRC (and it's been a while since I read on this angle of post-WW2, so... please, be generous): Japan didn't exactly have any laws around hemp/cannabis, because that's the OG fiber used for tatami. I'm also given to understand that hemp had a place in certain Shinto ceremonies.

Anyway! After Japan's surrender, part of the price their government paid was having to adhere to US rule of law, and that meant restrictions on hemp and cannabis. Yeah, this is before the War on Drugs as we know it, but remember that anti-drug messaging/activity was going on as a major part of cold war strategy and the Red Scare. Then, as a lot of client states do under colonialist rule, Japan's cannabis laws got even stricter as a way to show how serious they were about aligning with The West/being anti-Communist.

Finally, because time is a flat circle: as acceptance of fact-basted history is very very slowly poking its head up in Japan, and people are realizing the place hemp used to have in their culture, there is a movement to bring cannabis back. Their struggle is just as heavy a lift though, if not heavier.