I'd say it's likely parallel development, similar to how some countries drive on the right and others on the left, when the road rules were being written they just picked one, and didn't bother to see how it was done in other countries.
Writing a date as a set of numbers only is fairly new in the western world, I think, almost all of the old documents I've seen were written like 1st of January 1778
Yes it can be bloody annoying, but we honestly can't expect them adapt the system to suit us, both ways of writing it, even if you wanted to go yyyy/mm/dd it could work as well, are valid. It's not like the metric system where everyone knows it's retarded they haven't adopted it.