It's mostly about TV. Gillette being the sponsor of the Patriots stadium, and also one of the NFL's biggest advertisers, would much rather have all eight Patriot home games in Gillette Stadium, on regional and national TV, with their company logo displayed a million times during the game.
Tampa Bay, with a smaller market, isn't half as attractive to the TV networks and advertisers.
I doubt if the owners would go along with something as simple as a coin flip, and I don't think it was just a part of the normal schedule, since Robert Kraft stated how happy he was to have not lost a home game. If it had just been part of the regular schedule rotation, he probably would have said something about it.