The reason that the UN focuses on the US and Europe is that we are the developed nations. We are the West. We should be leading by example, otherwise why the fuck would the newcomers on the scene bother? They've not had the chance to enjoy careless pollution for decades like the US has, and you want to tell them that the US can keep fucking things over because it's not the biggest polluter? Seriously?
If the US were doing their part, it'd put more pressure on the rest of the world. It'd make them able to help other countries build clean infrastructure. Instead, they act as the black sheep, the dissenting opinion which developing countries can point at to justify them not doing anything. And despite all of this, China is in fact doing things. Pollution has become a serious issue and they're working towards solving it with billions of dollars a year. In twenty years, if all else remains equal, I expect China to be significantly more advanced in green tech than the US. Perfect? No. But no one's asking for perfection, just willingness to put the world's fate before your own petty politics.