The NFL is doing it because they're greedy enough to try to make more money. The billions they're making now is not enough for them.
I read the article and came away with a different impression. It looks like the NFL is doing this as a way to off-load their own NFL Network, which they’ve been trying to do for years. From what I’ve read about the matter, the NFL simply ran out of patience in dealing with all the disputes they’ve had with individual cable TV companies over carriage contracts and fees. They want to give up the network in exchange for a stake in ESPN because ESPN already has extensive coverage through its existing contracts.