I’m surprised the NFL hasn’t made the Super Bowl a pay event. It would be idiotic to do so, but the NFL meets that standard now.
They make way more from ads than they would from making it pay per view.
My guess is that it's tied to the regular season contracts from the networks. They might lose money on the regular season games knowing they can make it up with the playoffs and Super Bowl, even if they only get 1 every three years. I believe it's rotated among CBS, FOX and NBC.
Take the Super Bowl away and the regular season money would drop quite a bit.