I'm not a football fan, but it's nice to see such a national institution decide they have to get distribute their content on the small screen if they want to keep up with content people are willing to pay for E.G. Netflix, Amazon Prime, ect.
I'm betting its because their ratings are tanking right now. Americans generally became apathetic about the game when players and teams started disrespecting the American flag.
The NFL lost their audience and has to find a new one.
I think that and also most games are on Sunday when younger people are out and about combined w cord cutting. Very few people are aware that you can get HD over the air with a cheap antenna for free and even then, those games are generally the local team. A lot of young people follow players not teams. Lots of factors at play but the NFL wants eyeballs to get more ad revenue and getting on someone's mobile is a superpower for advertising purposes.