The problem with UMass is multifold. In football, they have never had a winning season in FBS since upgrading in 2012. Their record is 14-58. Fan support? This was their home attendance for last year: 12.1k, 9k, 7.7k, 10.7k, 11.9k. No success with no fan support doesn't make an attractive football addition to the AAC. Honestly, UMass' football ambitions are on life support as they can't continue as an independent over the long term.
What about basketball? In the past 20 years, they have been to one NCAA tournament (2014) and lost in the first round. Fan support? Here was their home attendance for last year: 3,857, 2,112, 2,042, 2,395, 4,015, 4,744, 2,004, 1,481, 2,243, 2,056, 2,476, 4,121, 4,194, 4,245, 2,641, 3,322, 2,953. How is adding UMass going to improve AAC basketball? They won't.
Although UMass is the flagship university in Massachusetts, they do not have the support that UConn does in Connecticut. Nobody in the AAC is going to support UMass joining the conference.