Here's a reason: some small populations like Norway and New Zealand are successfully going greener. This happens in part because our politicians are paying attention to their constituency.

It appears to me that many larger countries are not going green (in particular the US climate change denial) and part of the problem is that those at the "top" are very detached from those at the bottom.

Why would a single global political entity care more for the median person or the poor person? Care more for a forest, or a global oil company?