There's a giant elephant in the room that people tend to ignore (especially the MSM) when talking about this very subject. No matter what side of the argument you are on, the National Anthem protests are hurting the NFL.
http://boston.cbslocal.com/2016/10/27/nfl-ratings-national-anthem-protests-poll/
Fortunately for the NFL they have been able bury that matter and not have to deal with it head on due to the unprecedented craziness of the election which has affected ratings as well.
When confronted with ratings they can point to a handful of other things before they have to address that. Soccer gaining in popularity, baseball seeing a slight uptick thanks to Cubs, millennial consumption habits and aversion to male aggression/uncensored/sport/locker room culture, moms against the dangers of the game (concussions), etc...
It may not be the main reason, but it is part of the puzzle and it is a continuation in the politicization of sports, which is the bigger overarching issue. Not that politics and sports haven't crossed paths before. We all know that athletes have taken up causes good or bad. However, there has never been a time when it seemed like there is some sort of effort to start combining the sports and politics.
It isn't the act of kneeing, it is the coverage and all the opinions in the sports media that come with it. It's the fact that people who want to see some sports talk/highlights and escape all the nonsense on CNBC, CNN, Fox, etc now are getting similar attitudes and talking points on ESPN as well as other sports driven outlets.
People could have eventually moved on from the singular act of kneeing, and even accepted that more athletes would start to do it. What folks have not gotten over is the media wearing us out with it, they won't let us get over it.
In fairness to Colin, this trend of politics and all the mess that comes with it has been slowly creeping into sports and sports media for some time. I think the NFL threatening Arizona because of their law was an even bigger deal than Colin taking a knee, that was really kind of a sign of where things are going.
Going to be interesting to see how things go in the NBA. They moved the all star game out of NC so it's very clear that they are now moving towards being socially active. We will see if that has any kind of affect. Luckily they have some basketball lines that can sustain them. Durant and Warriors, Lebron at his peak, Dwayne Wade/Chicago, Rose/Nicks, post Kobe Lakers, KAT and T-wolves.