CGDA octave below cello
Wait... So theoretically, one could string up a piccolo bass in 5ths and actually have the same tuning as a cello? (I'm a cellist)
C G D A are actually the four most commonly used keys in music, so I concluded that it just made more sense to have those available as my lower strings because it makes a lot of music more practical to play than the way I had it tuned originally. Plus I have a D-Tuner on my electric, so I can still access the low Bb/B when needed.
As for the cello suites, I can put a capo on the 5th fret and have unison cello tuning, plus much closer to cello scale length.
I think most music is in those keys because those are the strings on cello/viola. And/or they were common from pipe organs, so when Montagnana and Stradivarius were developing their craft, they decided on those tunings? Or both, because if you look at Bach, he wrote in every key, but for cello, they're mostly in the cello keys (except for Eb 4th suite which really is a bitch to perform the entire suite, what an endurance test!). Vivaldi? Gonna be in the key of some open string on a cello/violin (or a relative minor of one of the strings, or relative major of one of the open minor keys, so Fmaj (from Dmin) and Bmin (from Dmaj) etc).
If you look at symphonic music composed by brass players, or for heavy brass sections, you'll find keys that are brass friendly, and nothing will be in C/G/D/A which is also a pain for cellists!
Then look further into modern music where guitars are involved - lots of music in the keys of E, F#min, etc because of the convenience of those keys on guitar/bass (4ths tuned instruments) and when there's a song in Eb (stevie wonder superstitious?) bassists go "can't we play it in E? I don't have a 5 string" haha
So depending on what genre you play, C/G/D/A are not always the most common key signatures.
I've played cello in more than a couple rock bands, and it becomes a real pain in the @$$ when a guitarist uses a capo and doesn't know what the chords are, then asks if I have a cello capo... No, there is no such thing as a cello capo... (and omg, please if someone has ever seen one, let me know?)