Lots of reasons, starting with the fact that US uses far more energy per capita than most countries. US also imposes its economic system upon the world that necessitates constant growth an consumption. US arguably being the wealthiest country in the word is also in a better position than most countries to actually do something about the problem.

Compare this with China which has an actual plan to transition off fossil fuels that it's actively implementing. China now leads the world in both nuclear and renewables by a huge margin, and continues to implement clean energy solutions at scale. If China is able to do this, then why is US isn't puling its weight?