Because BYU PARYS TOO MUCH.

At the time of the selection, BYU had...

Vastly better facilities,
Much Larger Stadium
Much larger Basketball Arena
National and even International Fan Base
Its own 2nd and 3rd Tier TV Distribution Network (which could have been coordinated with the PAC 12)
Academic Ranking in the top 65 USNWR, top 90 Forbes
Research Advanced Degree Programs, just 2 less than Utah and a significantly larger budget and endowment.

UTAH got invited into the PAC 12 because it was a STATE institution and not a private religious university.
IT HAD PRECIOUS LITTLE TO DO WITH THE ATHLETIC DEPARTMENT.

UTAH'S COMPETITION WAS:

Hawaii (too far)
Idaho (LOL)
Boysenberry State (LOL)
Northern Arizona (LOL)
Texas (wasn't interested)
Texas A&M (wasn't interested)
Texas Tech (wasn't interested)
Oklahoma (wasn't interested)
Oklahoma State (wasn't interested)
Colorado State (PAC 10 was not interested)
New Mexico (LOL)
New Mexico State (LOL)

Be realistic, the PAC did not want BYU because it's "difficult" for Oregon, Washington and Cal to accept any university whose Board of Trustees might believe in the ancient earth (4.6 byo) hypothesis as well as an ancient God to go with it.