My family has two hardcore BYU fans and we keep attending to support the team. The rest would rather do something else and at this point I can't blame them. There's no way I can promise them going to a BYU football game will be the most exciting three hours of their week.
So two of us attend instead of 4 or 6. Multiply that by a couple thousand families and you've got the difference between a sellout or not.
Even now it remains a pretty slim difference, but if the slide continues and more people hit their breaking point it could be a long road back to selling out LES.