...alcove on the East Coast? Before, most popular sitcoms — at least for the previous 15 or so years — were set either in anodyne suburban locations or in rural America.

Because that's where the creators and writers came from. This shouldn't be too difficult to figure out. Even Garry Marshall who set Happy Days in Milwaukee came from the Bronx.

CBS's "rural purge" of 1971 not only resulted in new shows set in urban environments (Bob Newhart in Chicago, MTM in Minneapolis), but spinoffs were also in urban settings -- like Rhoda in New York, Lou Grant, etc. Once the networks decided to go after the urban demographic, things snowballed. (Family Affair, which ran into 1971, and That Girl were good examples of the urban trend's beginnings, but of course shows like I Love Lucy, Car 54, and The Honeymooners had been NY based for years.)

Even Doris Day's show switched from a farm setting to an office hi-rise. The M*A*S*H spinoff of sorts Trapper John MD had the character far removed from rural Korea.

New series like The Odd Couple and Welcome Back, Kotter were deeply rooted in New York -- as Babalu pointed out the guys behind those were NY born and bred.