I think people who are surprised at the popularity of keeping NA native fish in Europe are people unaware of how common coldwater tanks are in Europe. In the US the first question people ask of you is "fresh or saltwater"? Euros ask if your tank is warm or coldwater. Once you commit yourself to a coldwater tank, adding NA fish greatly expands the colorfulness of the tank.
That's interesting, it seems to me that tropical freshwater is really all people are interested in here in the US. The only coldwater fish I ever see for sale are goldfish and koi. Now that I've discovered native fish this is kind of a curiosity to me considering the diversity and color of NA natives.
It's also a little strange that Europeans are interested in NA natives; Europe is a big place, and I'd assume there's a diversity of colorful native coldwater fish...