One thing that makes marijuana more complex is that, unlike was the case with alcohol, the prohibition is written into international treaties. Countries could just ignore or withdraw from them, or try to get them modified, but so far none seem to be willing to. Treaty obligations relating to banning marijuana trafficking are one thing that's complicated the Dutch experiment with decriminalization, and led to absurd situations, where coffee-shops selling pot to customers is tolerated, but that pot cannot legally come from anywhere (they cannot grow it, cannot buy it in wholesale quantities, etc.).
The reason other countries won't flout the UN Single Convention and other international treaties is...the US. They don't want to be branded outlaws by the US, with potential sanctions, banking restrictions, etc.
Sort of like the ancient Klingon proverb: "only Nixon could go to China", only the US can legalize (vs. merely decriminalize) drugs.
