The St. Louis question mystifies me, so I've been mulling it over. Personally, I was raised in the pop half of Kentucky, but I now attend graduate school in St. Louis and am soon to be married to a St. Louisan.
I don't agree with ibn centre's hypothesis that St. Louis "soda" usage is dictated by the transient student/young professional population. Most of us are confined to St. Louis proper, and the map shows a pretty large circle of "soda" emanating out from St. Louis. Furthermore, for every "soda" person who moves here, I'm sure there's a "pop" or "coke" user such as myself.
So the question I've been pondering is...what does St. Louis have in common with Milwaukee (another soda bubble) that it doesn't have in common with other German areas such as Cincinnati? My answer (and yes, this is a stretch), is the Lutheran Church.
Two of the US's largest Lutheran populations reside in these two areas. In fact, the Lutheran Church Missouri Synod (2nd largest Lutheran denom in the US) and the Wisconsin Evangelical Lutheran Synod (3rd largest Lutheran denom in the US) are based squarely in these two soda circles. The other Midwestern German populations are either more heavily Catholic (Cincy) or far more transient and diverse (Chi-town).
Anyway, that's my incredibly far-fetched theory.... the Lutheran Soda Conspiracy.
I have been thinking about this ingenious hypothesis. If does run into some trouble with the fact that the whole upper Midwest is Lutheran, but not soda-speakers. This map from the Glenmary Research Center shows the Lutheran distribution. So I offer this possible refinement: the soda bubbles emanate from German Lutherans.
I hope you will treat this with all the seriousness that it deserves.