Pop vs. Soda?

This map of generic names for soft drinks is fascinating. I can understand why "Coke" is a generic name in the south, with "pop" predominating in the north, but why the four disconnected islands of "soda"? Do they represent the remnants of some earlier wave of migration that was overwhelmed by later arrivals? Are there economic factors at play? The public wants to know, dammit! (via Aetiology one again)