I've always wondered, but never really checked into it. Maybe it's a German thing?
Seems like the same kind of thing as kale and mashed potatoes. Isn't that Irish? Colcannon? That's good. I love sauerkraut AND mashed potatoes, so I'll put them together one of these days.
From what I saw in Germany I would definitely say it is a German thing. They did the same thing with lamb as well