We can debate exact market split all we want, but honestly that's not even the big question. The real issue is what incentive does the B12 have to double up on a very mediocre media market (SLC) when they already have a team in that market? Especially when they can monopolize that market by sending their only competition back to the MWC.
Conference expansion is about dollars and eyeballs. Utah provides a very limited number of both of those, and at the detriment of BYU. The B12 would much rather get the entire market and only feed one mouth.