Think about the sorting use case:
Say you've got a bunch of files on your system with the date as the first part of the filename, what is more meaningful, sorting by year (2015-01-19_file), or by the date (19-01-2015_file). Clearly, sorting by year the files were created is more meaningful than sorting by the day of the month the files were created.
Edit: I completely misread your comment as saying that smallest to largest is more useful than largest to smallest, rather than the weird month-day-year thing we use in America. Completely agree an ordering by specificity is more useful, but going from least to most specificity makes it much easier to order things in a useful way.
Think about the sorting use case: Say you've got a bunch of files on your system with the date as the first part of the filename, what is more meaningful, sorting by year (2015-01-19_file), or by the date (19-01-2015_file). Clearly, sorting by year the files were created is more meaningful than sorting by the day of the month the files were created.
Edit: I completely misread your comment as saying that smallest to largest is more useful than largest to smallest, rather than the weird month-day-year thing we use in America. Completely agree an ordering by specificity is more useful, but going from least to most specificity makes it much easier to order things in a useful way.