Long answer?
Starting about a decade ago, the going rate for having non-power 5 FBS teams come and play at your place was driven upwards (+$1 million) by power-5 schools flush with cash. Large schools with big, on campus stadiums can pay teams like UMass (Florida), Western Kentucky (Alabama), and Arkansas State (Auburn) to come to their places on single game deals and still make a large profit.
By not owning their own stadium, and keeping all of the profits from a game, UM has effectively lost the ability to pay the going rate for these games. Hence them having to sign home and home deals with teams like FAU, FIU, Toledo, and Arkansas State.
Short answer, because someone canceled on us late enough in the process that, lacking the ability to pay a FBS school to come play us at Hard Rock, playing a home and home with a team like App State was the only viable option.