I think C being the peoples key may date back to the time when pianos or claviers did not have any black notes, so every tune was in the key of C. The joke was "why did J.S Bach have so many children? the answer being "he had no stops on his organ", stops being black notes, or sharps and flats. When these instruments were updated to contain black keys, Bach composed works for the Well Tempered Clavier, which would be the first tunes played in different keys.

Well that's a most interesting interpretation of events to be sure.

Just on the off chance that you're not joking, have a look at Ross Duffin's excellent book How Equal Temperament Ruined Harmony, which, amongst many other interesting things about the development of Western music, covers the circumstances in which Bach wrote the Well Tempered Clavier.