The NFL doesn't (yet) allow sponsors on regular NFL jerseys, but if they did, who'd you want as a sponsor on the Cowboys' jersey?
Since 2009, the NFL has allowed teams to have corporate sponsors' logos displayed on their practice jerseys as a way to generate more revenue. In the eight years since, the Cowboys have had a little AT&T patch on the left shoulders of their practice uniforms every year.
The NFL's permission still has not extended to the official game day jerseys, which is a bit surprising for a league that has repeatedly shown that making money is their number one, number two, and number three priority.