From an outsider's perspective, I feel like it might be a question of fan fatigue because, on paper, the NFL has done just about everything right in terms of running a competitive and entertaining sports league. On the football side, I've always been an "Iowa fan" rather than a "football fan" mostly because, unlike basketball, I find the game of football to be, well, a little stupid. As such, I've never really developed more of a casual interest in the NFL. At my peak interest level, I watched a few full seasons of Broncos games during the Manning run.
From what I can tell:
- The level of play is incredibly high.
- An remarkable number of games are competitive - potential game winning drives in the last ~3 mins of the game almost seemed commonplace.
- The parity is crazy - if your team sucks, just wait a year.
I mean, on paper, as a sports fan what more could you ask for? As a college football/basketball, NBA, and F1 fan, I feel like each of those leagues/series could learn a thing or two from the NFL in those regards...yet, people do seem to be getting bored with it. Chocolate pie is really tasty, but if you eat it every day, it would start to be just like celery to you, I guess.
The Super Bowl hype gets to be a bit much. I feel like the consequence is people - from hardcore fans to the "I watch 1 football game a year" types - need the game to end 64-63 complete with crazy plays and controversial officiating to be deemed a "good game". A defensive battle or any game that isn't really, really close will be deemed "boring".
Just my observations.