BYU fans want to see the game. If it is played at BYU you have a 65,000 seat stadium and close to a guarantee it will be on ESPN/2, while if at Utah State it is in a 30,000 seat stadium and possibly not on national television -- especially with the state of the WAC/MWC TV contracts.
So, if the game is at BYU almost 35,000 more people can attend the game and it will be on national TV. This is likely the largest concern for BYU. While Middle Tennessee State may not have a huge stadium, there aren't the same number of BYU fans in Tennessee as there are in Utah so the stadium size isn't as important. Also, since ESPN has the rights to Sun Belt games (unlike the MWC), there is a good chance that ESPN picks up the game, even if just on ESPNU.