National audience or even fan base size doesn't matter.
What they are looking for is selling rights to third tier games. This comes down to negotiating with pay TV companies in individual markets. Only on the Wasatch front does BYU command a marketshare that could force a cable operator to pick up the P12 network. If they pick up BYU can the P12 sell their network anywhere new that they can't sell with Utah?
Nope. BYU fans are everywhere, but not in high enough numbers to change market forces or land a new market.
Show me a market whose Phone numbers don't start with 801 or 435 in which BYU broadcasts get a non trivial rating.
In terms of selling that contract they decided BYU and Utah were the same so they moved on to other considerations. They made their choice now they have no reason to add the other school.
Never said culture didn't play in the original decision, just that it wasn't the huge thing you want it to new and it's not what is stopping them now. The reason now is return on investment.