;) Cultural standards. From Europeans I've seen DDMMYY, from U.S. it's generally MMDDYY (and Canadians are conflicted). Heck, even Wikipedia's ISO 8601 has a section Related Standards, detailing the country-specific standards (that may or may not conform to ISO 8601).

I suspect the major reason we have standards is because of TIMTOWTDI.

I'm also reminded of XKCD's comment on standards.