You mean the NFL can’t afford to buy 4K cameras and equipment?
The NFL isn't the one paying for them, it would be Fox etc. who pays for them.
Think of it this way, the size of the audience watching would be the same whether it is in HD or 4K. Advertisers aren't going to pay higher rates because the game is in 4K. So it is all cost with no return from the perspective of Fox.
I think they'll do it eventually, but they're in no rush. When they need to replace equipment they'll replace it with 4K stuff, but they aren't going to retire working HD gear for a while. Having 4K cameras makes sense even in an HD production for the zoom, but beyond that...
I think 4Kp120 is the point where it makes sense to really go into 4K for sports, because that adds more value than the increased resolution (at least if I had to choose I'd take 720p120 over 4Kp60) just like HDR adds more value than the increased resolution. The problem with 4Kp120 is they need TVs that can accept that as an input, which almost none can (the 2019 LG OLEDs and a couple others are it so far) and you'd need set tops / clients with HDMI 2.1 which so far there are exactly zero.