The US are late to the party. The NFL make a butt-load of money already, they don't necessarily need more but they'd like a bigger butt-load. Jersey sponsorship is a delicate issue with some in the US. The NBA has shown how it can be done tastefully, the MLS gets away with it because a big sponsor on a jersey is just how soccer goes about its business.
It will happen eventually, give it 5-10 years and most will have some form of ad I guess. They won't be able to ignore the easy revenue. A team like the Jags would put one on there tomorrow if they could.