In a word, California. Because of the geography and climate there, smog forms very easily in large parts of the state. So, California emissions regulations have

In a word, California. Because of the geography and climate there, smog forms very easily in large parts of the state. So, California emissions regulations have been focused - since the mid-20th century - on smog precursors. NOx is a major smog precursor, so they have a very good reason to minimize it. This is where we get into politics. The US as a whole wants to adopt somewhat tougher emissions regulations, and car manufacturers want to minimize the number of different standards they have to meet in a single market. It turns out that everyone's on board with "just make the California standards the national standards" and so that happens. Then California tightens their standards again and the cycle repeats. Europe doesn't have a comparable political subdivision with such a high natural propensity for smog formation and the ability to make their own standards. The regulatory differences emerge from a combination of geography and political systems.