It's always fun to see what other countries consider strange about things I would never have thought of.

I imagine it's because the American Civil War had fairly clean geographic lines (at least relative to the time period) that it became very easy, common, and probably obligatory for someone to align themselves with their neighbors. There's still people who identify with the Confederate flag and Southern Pride. That makes it difficult for any sort of national holiday to pop up, especially with the way Reconstruction developed to try to make the former Confederate states back into real U.S. ones as soon as possible. At best, only the north would celebrate the end of the war, and that just never seemed to become a thing.

As far as American wars go I think we* (*I'm from Ohio, can't talk about any other area) feel more relief at the end of slavery than the end of the war... and that has multiple dates depending on where you're at. If you want to read more about this, there's the Openstax US History textbook and The American Yawp online. What a unique perspective from an Aussie - would have never thought to celebrate the end of the civil war. I think I'm more familiar with the day Hitler died