Because it matches the way we say and write unabbreviated dates - November 21, 2009.
Neither day-month-year nor month-day-year is any more logical than the other; they both make sense for different reasons, and they're both less ideal than other alternatives. ISO standard dates > *. 2009-11-21. Unambiguous and sortable.
I am an American, and am used to our date format. But I would still say it is a terrible way to format a date. Why is the least significant part of the date in the middle? It makes no sense at all. And of course many people drop the second comma in sentences like "On November 18, 2009, I took out the trash.", which then grates on me because I am a grammar Nazi.
In any event, yes, ISO-style dates are clearly superior. As an extra bonus, they are more portable across languages than dates that spell out the month (and of course if you use a number for the months, in other dates formats it is ambiguous which part of the date is the day and which the month, unless one is greater than 12... ugh).
This format is decent enough if you use a 4-digit year (because 2-digit years are
stupid) and one that I sometimes use (although, as stated above, it fails hard if the reader doesn't speak your language).
However, its bastard child the DOD
date-time group is just ridiculous. The time is integrated into the day component for some reason, with no punctuation at all to make clear what is happening. I am convinced that the DTG format was invented as some sort of horrible joke. On the plus side, it does concisely provide the time zone.