The New York metro area is vastly bigger than what the Sox have to work with. The Yankees share that market with another team that is leading the National League East. It is not like they have it all to themselves, like Boston.
Rhode Island, just down the road from Boston, is about half-and-half between Yankee and Boston fans. That's a real failing by the Bosox. Rhode Island should bleed red, and lots of their fans are following the Yanks instead. The same goes for Eastern Connecticut.
Eastern Connecticut is Red Sox, the western is Yankee.
Do you have any clue what the total population of New York is within 50 miles of downtown? If you did then you wouldn't try to compare New England and New York. Massachusetts has about 6.5 million people. Manhattan alone has a greater amount than that. Throw in the surrounding areas and the amount is a few times that number.