The New York metro area is vastly bigger than what the Sox have to work with. Take the 5 boroughs, Long Island, western CT, New Jersey, the surrounding NYC counties and the area is gigantic compared to NE. You could put another team in that market and the fan base could (not would necessarily) support it
Hey, it is what it is. The Yankees also make other teams better by having to compete with them.
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.