Why? They get no TV revenue for playing at BYU, and the P12 contract is already set so they don't get more for playing BYU instead of FCS at RES. But, assuming the stadium is over 1/2 full for each FCS game, they earn more in home revenue against the FCS. So purely from a monetary standpoint, it makes sense to do the two FCS instead of H-H with BYU.
Also, P12 football games are on the P12 national network. There are no "regional TV" FB P12 home games.
As far as exposure, I don't think this is a primary consideration. If Utah can win in the P12 they won't hurt for exposure. Losing on ESPN doesn't really help, I don't think.