I think it’s just because the Ultra is so big/wide.. the regular bands just don’t look wide enough. I know the Ultra bands are wider, but Apple does not make many options for the Ultra and the ones that are out, are kinda “loud”, ya know?
Last year I tried my Spiedel watch band on the Ultra and my wife and I agreed that it looked fine. That said, I didn’t buy the Ultra then but I may this year.