cheap widescreen HD TV's have combined to take a significant toll on live sporting event attendance nationwide. It's not just a BYU football issue. Add to these things the fact that BYU has produced mediocre, disappointing, less than entertaining football teams in recent years, and you have a serious problem.
I think cultural changes are partially to blame as well. Fewer and fewer young people care about sports of any kind...much less a sport that costs a fair amount of money (for a college kid) and a significant investment in time to watch live.
I think the BCS, and now the whole G5/P5 thing, has taken a serious toll on many people's college football fandom, too. I feel my own interest in college football diminish with every passing week under the current system. I elected not to renew my season tickets last year. And, where I used to watch 4 or 5 college football games each Saturday, I now only watch BYU's game. I'm ashamed to admit that I typically don't even watch BYU games on live TV anymore. Instead, I usually spend Saturdays during the fall hiking or fishing. I'll come home and watch the game on DVR. If we lose, I don't bother.