Ratings are down for numerous reasons. But the biggest reason, by far, is that the NFL is not a good product. The games are boring. The commercials are endless, as are the flags. There is zero atmosphere for most games and the teams/owners absolutely gouge their fans. At the end of the day, people aren't stupid. If they are no longer having fun, and if they feel they aren't getting value for money, they will find something else to do.
Personally I haven't cared about the NFL for years now, and whenever I do try to watch, I find that I simply can't. This league has somehow managed to take the greatest football players in the world and create a game that is absolutely dreadful to watch.