IMHO The ability to choose to live in a high income region (or more specifically a cosmopolitan city) is one of the core characteristics of what it means to be middle class.

Partially, that's because increased self determination is part of being middle class. Partionally, that's because the ability to participate in culture (art, music, education, multiculturalism, etc.) is part of being middle class; and those opportunities are highly concentrated in the cities.