Fortunately, here comes . . .
Science!™ to the rescue!
Using data from the Sonic website, and the 2010 census numbers, I have calculated the number of Sonics per million people in each state. (To be fair, "calculated" overstates things a bit.) The pattern is centered on Oklahoma and Arkansas, and reaches much farther east than I had realized. So, it is perhaps better characterized as a south-central phenomenon. My apologies and respect to @ElenaMorning.
Here's a visualization, created using Targetmap:
Feel free to refer to this map when deciding where to live next.