schools like Utah. The main reason they don't just take the conference champions is because that would make selective exclusions like that impossible.
A school like Utah participating in the playoff would cost advertisers, and ultimately the P5 conferences, a lot of money. A lot of people would still watch those games, but not nearly as many as a Michigan, Ohio State, or Alabama.
The only way a smaller market team gets selected is if excluding them would delegitimize their crowning of a "national champion". For that, the team's resume would have to be perfect. Utah fell from playoff contention the day they lost to USC. That loss gives the committee a much appreciated reason to select someone more profitable.