doesn't always have a full stadium - even during their recent juggernaut dynasty years - to the point that Nick Saban issued a public rebuke to the fans, telling them that they need to come support the team.
Nowadays everyone has a widescreen TV and 100+ channels available in their home, and central air conditioning, and can sit with a laptop and get some other stuff done while watching college football all day long.
Young people have their smart phones and social media and video games and aren't nearly as interested in going to a football game as in decades past.
It isn't just BYU - almost everyone in college sports is dealing with declining attendance, to one degree or another.