;) 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).
in reply to Re^4: sorting dates in YYYYMMDD format
in thread sorting dates in YYYYMMDD format
I suspect the major reason we have standards is because of TIMTOWTDI.
I'm also reminded of XKCD's comment on standards.
Quantum Mechanics: The dreams stuff is made of