Because there are about 20 people who will pay all that money for the NFL games (do these people not get Fox, CBS, ESPN and ABC?), while there are probably tens of thousands who would use SlingPlayer. The NFL stuff would also be much more limited viewing -- only NFL games, which you can't just watch and watch and watch -- versus generic TV, which someone could watch like crazy.
In other words, the SlingPlayer would kill the 3G network in some areas, while the NFL app will use less data than two people checking e-mail. If you can't tell, I think Sunday Ticket is a complete waste of money unless you're a die hard out of your team's market or a sports bar. Even then, I wouldn't want to watch a game over a phone. If you're talking audio, sign me up.