KOUU 1290 in Pocatello, ID is AM stereo. It might help to check your facts before making a grossly incorrect pronouncement. A web page with current AM stereo stations is available hereI had only considered the one I worked for, any of which would gladly sell you the stereo equipment they wasted money on. When I was working in Chicago we had two all-polka stations, so it shouldn't surprise me that there are still some AM stereo station in operation.
Kinda' like the "cave man" commercials for Geico.
Consumer equipment to enjoy AM stereo isn't as widely available as it should be. KOUU has a very successful country format in Pocatello. They tout the stereo capability, but it isn't a necessity to receive the broadcast. I'm not surprised polka isn't a real successful format. Chicago may have a big Polish community, but even they aren't going to listen to that format 24/7. It's unlikely anyone else would even bother.
BTW, there is a great Polish buffet called "A Taste of Polonia". It's my favorite lunch spot when I'm in Joliet/Countryside for work.