In other words, why does the aeolian mode (i.e. the “first” mode, and basis of the standard major scale) begin with the third rather than the first letter of the alphabet when there are no sharps or flats? DuckDuckGo could probably tell me but I’d rather wonder out loud here.
My total guess is that when Western music was being codified, most songs were written in minor keys, and so at that time ionian–not aeolian–was considered the “first” mode.
Some of my music posts have received comments from people who seem rather informed, so maybe someone can fill me in.