I wonder why no-one has ever thought of marketing it in Poland.
You give the answer yourself:
a speciality of the Detroit area
I've been in the US many times and I drink Ginger Ale quite often. In Mnpls, for example, I never came across Vernor's Ginger Ale. If they don't or hardly market it outside the Detroit area, why on Earth would they market it in Poland? It's too specific, I guess. Some things don't get marketed outside the immediate surroundings of the producer. And that is normal. I never wondered why they don't sell "Herb's Barbecue Sauce" (a Bloomington MN producers) outside of Minnesota. At least, I never came across it anywhere else in the US, let alone Europe. Too bad, it's a great BBQ sauce in many varieties.