ISO ordering would make expressing time in AM/PM ambiguous, i.e. the format string "2:05PM".
We have a perfectly good, unambigious date and time format. It's true that many users are deeply committed to ambiguous formats, but we should at the very least be trying to dissuade them from using them by making ISO formats the default.
The format string "4 minutes after 12", is unique, but it's not clear that I could turn "34" into "4". Similar for the hour (12 is now 2).
That would also break the "Print this exact time in the format you're currently using" method of creating a compatible time format string.