It makes sense because Boston, NY and Philly are 3 of the biggest markets (and with 3 of the biggest fan bases) so putting them on tv means $$$. An argument could be made for the Dodgers/Angels, but being in the Pacific Time Zone, not many people in the rest of the U.S. are willing to stay up til midnight or 1 a.m. to watch the rest of the game.
I went with popular because the Cubs have a gigantic fan base but they aren't very good which hurts their popularity.
And do they ever do Sunday Night Baseball on the west coast? Sunday it's a bit easier to do a 5pm start out west than it is during Monday or Wednesday night baseball.