1) Too many new schools are bringing down the FCS average? So a school jumps to FCS in 2012, and the fans are excited --- but after 5 years of playing Georgia State, they lose interest.
2) Conference expansion & rivalry dilution --- see Maryland & Rutgers. Doubt that is helping the average for Big Ten games.
3) Some schools/conferences are going through a slump --- see the Pac-12, with UCLA, Southern Cal, and Oregon.
4) With the growth of secondary Internet/StubHub sales, it has become way too easy *not* to buy Season Tickets. Too many schools are living in 2004, and they really haven't adjusted the price & value of Season Tickets.

One of the key selling-points of Season Tickets? The convenience. All your tickets, at once, all together in an envelope. But you know what's also convenient? Grabbing your phone, and buying a below-face ticket in 15 seconds.

So instead of selling 30k Season Tickets, maybe the school sells 21k --- and the people who don't renew are quite content to pick & choose 2-3 games to attend (instead of "attending" all 7 games in the Season package).