Much of the TV/internet sports media is based in NY. The people that run these networks are NYers. Yankee fans. Knicks fans, etc. Plus NY is the biggest advertising market.
Boston and NY sports markets overlap. There is a competition for some of the market. Hence, Boston teams get negative spin on TV/internet outside of Boston.
A big advantage the Yankees have is their Yes TV Network, which is plugged into many sports cable TV packages around the country. Whether or not you want it. Like ESPN. That's why Yes is a billion dollar network for the Yankees. They get 100% of its revenue, because it's a "local" channel. Local?