I personally boytcotted the NFL a few years ago because the Washington team won’t change its name, and I won’t even consider watching it again until the league forces them to change their name or kicks them out. So, there’s a single data point for you.
More generally, maybe people aren’t interested in watching a product that clearly has not evolved since last century, where racial slurs are tolerated as valid mascots, where health risks like player concussions are largely ignored, where the players themselves are tightly controlled and forbidden from expressing any individuality, and where misogyny is on full display and violence against women is tolerated among its players. This stuff just doesn’t resonate with as many people anymore.
There is, of course, also the issue that nobody can afford to go to a game anymore to get attached to a team and, even if they could, franchise players are rare, so the team you get to know is completely different a year or two later. Also, the Patriots won for years by cheating. That kind of makes it all seem pointless. Then there’s the issue that was brought up repeatedly in the comments here so far that the games are boring; Colin Kaepernick taking a knee before games is literally the most interesting thing that has happened in the NFL this year.
Also, the NBA is just way better now. Players don’t get flack for expressing themselves, and aren’t treated like disposable means to an end. The NBA is loaded with talent, with superstars with interesting personalities and stories, as well as long careers. The games are also way faster paced. The average ticket price is about 60% that of an NFL ticket, too. In case you think I’m being subjective about this: here’s some data.
It is also probably a factor that the media surrounding football is terrible but the media surrounding basketball is entertaining. Maybe if Shaq did a Shaqtin A Fool segment for the NFL every week it would help their ratings. He probably has too much integrity to get involved with anything as shady as the NFL, though.