I'd assume it's a combination of factors, not least of which is three straight 8-8 seasons. A lot of these tickets were probably sold in the preseason.
But demographics and price probably do have something to do with it. I bet a lot of foreign fans, mixed with the increasing ticket prices, makes this happen as well.
Think about it.... 49ers fan.... you have to pay $630 for your family of four to go to a game, and it's a one-time thing. You won't get this chance for another couple years. If you're a Texans fan, at least not another 4 years, if not 8.
You're gonna splurge. $630 isn't unreasonable for that experience once every half decade.
If you are a Cowboys fan, you are paying $630 a week. That wears on you when it's the #1 highest price in the league.