Reasons it's declining (and these have already been mentioned by many):
1. Young people don't watch live TV. If it isn't on their phones, they aren't watching
2. CTE cover up
3. Domestic violence and other issues involving NFL players, which the media feeds upon
4. Uneven punishment for players and teams by Roger Goodell
5. Dominance of Patriots for almost two decades now-not good for business. Supposed to have parity
6. Donald Trump's vendetta against the NFL. HIs comments have stoked racism towards players.
7. League has overstretched itself with new stadiums, while income has been declining among most Americans (most can't afford to go to games)
8. Players taking stances like anthem hasn't been viewed well by majority of public
9. I could keep going...
I'll be the tin foil hat guy: because the Pats continue to cheat. Read Spygate by Timothy O'Leary and you'll understand