They had rights to the game but had very limited distribution nationally or streaming. BYU was able to work a deal to simulcast that game and paid Eleven Sports Network for the privilege.
This year, NESN re-upped for the regional broadcast rights and FloSports won the bid for rights to all UMass home games. They also have nationwide streaming via subscription for their content. UMass and FloSports are just doing what is best for them--even if it means a couple of us dropping a $20 to watch. The 4-game series is over after this game anyhow, and I don't know that it gets re-upped so each side has little motivation to budge.