Not all "Nationally-Televised" games are equal. You're confusing "exposure" with "availability." Games on BYUtv are high on availability but low on exposure. Games on ABC are high on both.

Other than the Florida State game (which drew more than 1M viewers on ESPNews) Boise has played a bunch of ESPN2 games in the 10pm late-night timeslot. Most broadcasts attracted fewer than 600K viewers, including the BYU-Boise State game. 

BYU has played two games on ABC over-the-air broadcasts and ESPN primetime broadcasts that attracted 1.6 to 2.7million viewers each. Huge difference in exposure.