The Hungarians (and presumably the Chinese) have their own just-so stories about why YMD is the most practical order: you use dates mostly when you sort heating bills, invoices, or even fruit preserves, and in all those cases you need to sort by year first.
Australians will tell you a tale about how "day before month" is human-scale, while "month before day" is not. Heck, ask a German why the decimal comma is superior to the decimal point, and you'll hear plenty of practical-sounding reasons. French carpenters will tell you how they use decimal units because those are much more practical than inches. British people will explain how left-hand traffic is the only practical choice (6a41ef).
People are much better at confabulating reasons for sticking with conventions than they are at telling practical and impractical conventions apart. Indeed, if there was one obviously practical choice, there would be no differing conventions for ISO standards to reconcile.