It's because they're a borderline JUCO that has no fanbase or media markets of note. Their stadium seats 5000 less than the lowest BCS conference team(Cincy). They're currently increasing the seating capacity of the stadium, which will seat 53,000 when complete in time for the 2013 season. According to Nate Silver, they have a bigger fan base than TCU. Boise State is the #41 highest seller in collegiate apparel (for the first quarter of 2010-2011 fiscal year) as well. Academics is the excuse I hear the most, but frankly I think it's far more likely about geography. If they were located closer further south, I think they'd have a better chance. According to Nate Silver, Clemson has the 10th biggest fanbase in the coutnry. Nate Silver is a god and his word is gospel.