The size of the market has no bearing on the success of specific formats. What matters is the percentage of people in a market... any market... who like a specific format.
Due to fragmentation within alternative rock, what we have are several distinct varieties, much of which does not overlap. The result is that any specific variety of alternative rock can't get a high enough share to be a sales success.
Country in NYC is the victim of nearly zero lifestyle presence and only a small partisan group. Advertisers just don't buy stations that are 15th or so in their target demographic.
There is also no Regional Mexican station, despite there being about a million Mexicans in the market. Not enough of them would listen to get a big rating and, thus, sales.
And in general, the increasing percentage of Blacks and Hispanics makes rhythmic music more prevalent. When a major group is so large, it can push tastes. In fact, more than half the market in 18-34 is Black or Hispanic and that pushes what is exposed in clubs, festivals, street fairs and the like. New York is a rhythmic town now, not a rock town.