I've loved football for the longest time. But I honestly believe some of the biggest factors contributing to the decline in ratings have to be predictability. Every year everyone knows New England is going to be at the very least in the divisional round if not the Super Bowl. It's the same thing every year and I think people are generally sick of it.
Also, the officiating is awful and biased. It pisses me off to a point where it's turned me off from the game.
I used to sit on my couch all day Sunday to watch football now I only watch the Bills' games. I think, for a decent portion of people, that the anthem protests were the last straw.