Equal temperament is NOT based on C and can't have anything to do with it.   The point of equal temperament is to make all keys sound the same, so if they don't there is something else at work.

One "something else" is mains electricity. The civilized environment is permeated by mains hum, which is A flat (50Hz) in most of the world and 60Hz (no standard musical pitch) in the US. This often made playing British pubs before the smoking ban a nightmare, as they needed huge loud extractor fans to keep the air even marginally breathable, and you got a thunderous A flat drone under whatever else you were playing. Since most Scottish music is in one to three sharps, the effect was grotesque.