In regards to why it hasn't been copied, I have heard that part of the problem is that the UN officially supports the War on Drugs. European countries are reluctant to break ranks with the UN in this matter, while progress at the UN itself is held up by conservative countries such as Iran.
Saying progress in the UN is held up by Iran and such sounds nonsense to me. The US is also pretty conservative and has way more influence and power inside their buildings.
I only have a friend's word for this (who works with drug policy), so if you know more please share. Obviously the US drug policy is quite varied, with full legalisation of marijauna in some states. The federal response to the opioid epidemic seems to be harm-reduction based rather than purely punitive. That said, this is internal politics, so what the US says and does on the world stage is not necessarily going to be consistent with this.
I don't think it's just Iran. I think the UK is firmly on the side of the WoD too.
And yes, the UNGASS is a big part of the problem, it sets the international direction and it only meets every 10 years. When it does meet it puts out a statement, and these statements so far have been utterly silent on any and all harm reduction measures, instead sticking to the old rhetoric about prevention and enforcement.
