An interesting issue, Will.

I'm frequently intrigued by the question of why tunes [& songs] are written in particular keys.

For songs, I'd guess that the composer would set the key according to their own vocal range - yet as we all know, most people, tho' not all, performing it will change the set key according to personal voice range.

My guess for tunes would be that it's the particular instrument that may dictate the key.

Other suggestions?