Although most US cities are cosmopolitan to one degree or another, New York is particularly noticeable due to the larger population, thus making the diversity more obvious - and the white people alone are remarkably diverse in themselves, unlike in most other American cities where they're relatively homogeneous, so it's possible to fall back on Monochrome Casting and still feature many unique characters. Very close to every single ethnic, racial and religious group is represented to some degree or another on the streets of the five boroughs, and nearly every language spoken on Planet Earth can be heard there. Further justification for this is New York's diversity.

In addition, New York City is the most populous metropolitan area in the United States (and among the most populous in the world), possibly justifying the frequency with which events of great significance occur there in fiction. There is a reason for this: the skyline is just so darn recognizable. The bias is especially obvious when characters speak about specific parts of New York casually ( everybody in the world knows which subway train you have to take to get to 115th street, right?), while the entirety of Middle America usually consists of about ten distinct places, or when any group of people naturally includes a Jewish person, because isn't one eighth of the population everywhere Jewish?

Javan rhinoceros