Because in short, it's alphabetical. It will always be in order by year, then month, then day. Literally like how a clock goes HH:MM:SS it's the same thing as YY:MM:DD the right side ticks the fastest. It's in order by hour (year) then minute (month) then second (day). SAME SAME WHY NOT
Because for 99.99% of all situations, you'd already know what year and month it is, so the most readily available piece of information should be the day.
If you already know the year and month why write it. ISO or month day are the two most reasonable. You need to zoom in not give yourself a list of options and then randomly pick one later.