I've heard this question a number of times from AHT readers without a Sonic in their area and from friends and coworkers here in New York City, where the nearest Sonic is 60 miles away (Waretown, New Jersey).
Over the weekend, I ran into one of the writers behind the two-folks-in-a-car Sonic commercials and asked him if he knew.
He said that it's basically a better deal for Sonic to buy ad packages on a national basis on cable TV, which is why all you Sonic-less folks see them anyway. An added benefit, he said, was that this strategy has coincidentally built up hype for the drive-in burger chain whenever it enters a new market.
And now you know.
Full disclosure: My sister manages a Sonic.