Käsespätzle is popular in Switzerland too. It's very well known except sometimes they call it Chnöpfli / Knoepfle / Knöpfle or Spätzli. It could be that they'd heard of it under a different name? Spätzle is even eaten in Hungary, eastern France and Serbia under local names.

You can easily find it on the aldi-nord.de website under Spätzle. I mean, it's possible that there are products that are on sale in local supermarkets that people have never noticed, but then is it really that the diets of the North and South are so different, or is it that some people don't like variety or trying new things?

As for Matjes, that's pickled / brined Herring. It's no surprise that people far from the sea don't eat it. But, pickled herring is super common. It's popular in Germany (apparently only the north), Netherlands, nordic countries, etc. Slightly different versions are popular in the parts of the UK near the ocean, in Russia and Ukraine, even Canada. In fact, it's pretty common in Minnesota in the US despite them being far from the ocean simply because they had a lot of immigration from nordic countries.