NFL is suffering for the same reason NASCAR started suffering in the mid to late 2000's and are now trying to attach themselves to a younger audience and change their system: They moved to far away from their core audience and alienated the large swaths of fans they did have that attended with horrible rules changes. NASCAR has altered their business plan by partnering with Monster Energy Drinks and trying to focus on garnering a younger audience through making the races more of a spectacle. For instance, at the Brickyard this year they had a huge concert that featured young artists along with a number of other things associated with Monster Energy Drinks (trick motorcycles and trucks, other ideas geared toward younger audiences).
Football is suffering from the those issues but mostly with the great coverage you can get from ESPN, Fox, etc and not have to leave your own home. Would I rather pay my regular cable/satellite bill which is probably $100 a month or pay over $100 a ticket to attend a single game?