The current problem is that NBC is the only network that has streaming rights for the NFL. This is basically just an artifact of their deal with the NFL. Most likely when it comes time to renew contracts that exclusive clause will be marked out and the NFL will allow all stations that have rights to broadcast games online.

The NFL is pretty much the trend setter in the sports world when it comes to media. They were the first to figure out the right way to do a TV deal back in the early 70s, the first to start their own network, etc. It's only a matter of time.