I seem to remember reading that both Monitor and NBC NIS had issues selling those programs in the major markets including NYC, which had a lot to do with their demise (Monitor had a long run from 1955 to 1975, but apparently the larger markets were dropping it in the latter years). I'm sure there's some here who either worked in NYC at the time who could offer more clarification as I may not have this story totally correct, that was a long time ago, but that is what I remember...............
As far as today goes. Given that NYC is the #1 market, and all talent is unionized unlike most in medium and small markets, the cost to produce a local show with all unionized on air talent, plus unionized board ops, call screeners, etc, probably makes the syndicated shows far far less expensive to air, thus increasing the profits substanially for their respective station. Bottomline, radio is a business and making the largest possible profit is what their goal is, just as it is in any business or in industry. So in that respect, the larger markets may be at a disadvantage in terms of doing local programming, even though the talent is excellent and the cream of the crop, no one can afford to use them.