Last year Eleven Sports Network held rights to the game through their partnership with UMass. They have limited cable/satellite exposure and even less in online streaming so for them selling the rights must have made sense for what BYUtv would pay for the game.
This year NESN/FloSports bought the rights to all UMass home football games--NESN is the regional carrier and FloSports is a nationwide streamer of sports content. By nature of who they are, they would be less motivated to sell rights to one of those games to BYUtv than Eleven Sports Network was last year.