Because C is really the most basic piano key: all white notes in C major! So probably the first key anyone learns on the piano.
I play a B/C button box but don't play in either of those keys all that often: have to go with the fiddlers who seem to prefer G,D,A. The box (piano accordion or button box) is possibly the "oboe" of the folk orchestra, to be used for tuning to: as I tell people, can't change my tuning unless you've got a screwdriver! (And wouldn't want to anyway!)
Sorry for thread drift!