cost for tickets (especially for the good seats) is much higher, HD television, night games to accomodate ESPN, Thursday/Fri night games (conflicting with work, school, high school games), lackluster performance, Bronco not coming across as very fan-friendly at times, etc.
There are a multitude of reasons why attendance is dropping and it's not just BYU. It's a nationwide-wide problem.
Will winning games change that a LOT? It doesn't appear so since we didn't sell out Houston or Virginia (which was an afternoon game on Saturday. Go figure) immediately following a road drubbing of one of the most storied programs in the nation in front of 100,000 fans. But BYU is trying to maximize revenue so they'd rather have some seats open than go back to selling them all for peanuts which is what made it so easy to sell out for decades.
It's a lot more complex than "either side" wants to admit.