Someone mentioned up-thread about viewing habits and I think that is something that isn't getting a ton of publicity, but could be a serious factor in the ratings.
The NFL basically relies on people to wall off a MINIMUM of three hours every Sunday to watch a game. A huge part of their business is getting people to spend 7+ hours of their Sunday to watch multiple games. I do this all the time; and for whatever reasons it feels different when it is football. If I spent at least three hours every Sunday watching The Simpsons, or Quentin Tarantino movies, I would think that was obsessive and strange, but watching football? That is just typical, American behavior.
In the DVR/Netflix/Binge watching era; people don't build their schedules around television programming anymore. If you watched 24 ten years ago, chances are you cleared out the schedule on Monday nights at 9 p.m.(or whenever it was on) to watch 24; that was just what you did. Today; you can just set your DVR, go out for a few drinks after work, and watch it when you get home, regardless of the time. Shows on Netflix, Hulu, Amazon, HBO, etc. are specifically built around the idea that YOU decide when the show is on, it isn't dependent on people tuning in at a specific time. The idea that your schedule is built around watching a television program at a specific time, isn't really a part of a lot of peoples lives anymore; except with the NFL. People are no longer in the habit of doing that in their day-to-day life, so asking them to do it on Sunday's during the NFL season is more difficult.
One other thing is that over the last couple of years the NFL has been uploading these great, 7-10 minute highlight packages of each game, on Youtube. I for one know that if I miss a key game, I can immediately go online and find all of the important moments and big plays in a HQ format. The days of missing a game like Seattle vs Houston and having to subsist on whatever 2-3 minutes they show of it on SportsCenter are long gone.