Pre-market saturation. There's no Sonic's near you now but the idea is that there will be. A couple reasons come to mind on why they are using this strategy:
- Advertising is cheap enough - they aren't losing a ridiculous amount of money
- There's a natural human tendency to want what they can't have. The commercials are meant to stir up interest, and combine that with a lack of availability causes people to desire it more.
- The best advertising is word of mouth - you want people to talk about your product and pass the word along. Everytime I watch a Sonic's commercial with a group of people someone points out the fact that the nearest Sonic's is miles away. This is good.
In-n-out used a similar marketing campaign which was highly successful. Don't be surprised when theres finally a Sonic's near you!