Mobility

In places like Europe or China or India, places that are old, where the ability of the population to move very far was constrained, different cultures and languages arose. However, the United States, built from a conquering nation that used manifest destiny as its mantra to go from east to west, grew in a span of less than a few hundred years (if we count the first appearance at around the 1600s to its westward expansion in the mid 1800s).

It's perhaps no surprise that the locations that are most distinctive are the oldest. New York City, Boston, and to a lesser degree, New Orleans, where displaced French lived.

Given the greater mobility (and a unified country), there's far less regionalism than in Europe, India, and China. Effectively, Europe is how India and China would be if there hadn't been some attempt to unify the entire country, as a confederacy of different cultures and languages.

These older countries deal with both old and new. A language and culture to preserve, yet the mobility to move to anywhere in the country. Interestingly, this has not had the effect of moderating the culture, because to do so would mean the culture or language are not that important, and people seem to cling strongly to that.

Indeed, despite a kind of pressure, various Southern dialects in the US survive (as do Boston and New York accents) with far less cultural pride evidenced in states of India or China.

Mobility has something to do with all of this, but it's not everything.