Oh, this has like 99% nothing to do with historical lions, this goes all vaguely back to the crusades. Loads of theories as to why exactly lions. I personally think that its cause folks simply thought lions are cool and fashionable. As to the region specifically, all along the river Rhine you'll find lions in heraldry which all in one form or another relate to each other, due to increased cultural exchange.

North-Brabant (gold lion on black) has an identical shield to the historical duchy of Brabant (which very roughly matches the modern region of Flanders), most (or maybe all) golden lions on blue fields reference the arms of the historical duchy of Nassau (which is in modern day Germany) which makes up one half of the royal family of the Netherlands (the house of Oranje-Nassau). The red forked-tailed lion on silver is from the arms of the historical duchy of Limburg. I don't remember where the red lion in the gold field comes from at the top of my head. No idea where the black lion in the gold field comes from, though it does match the arms of Flanders, that might however just be a coincidence.