'Civilized people simply don't exploit and murder other civilized people' is what everyone thought at the time.
And, by and large, this was true - in a sort of 'honour among thieves', civilizations tended not to exploit other civilizations - preferring to exploit the vast majority of the world, all those other people they deemed 'uncivilized' - in fact doing so was the very hallmark of being civilized.
To be civilized was to hold dominion over lesser 'others'.
So during WWII, civilized people in the world were greatly horrified to learn that the civilized Germans were forcing the civilized French to provide involuntary labour, 'just as if the French were but mere darkies in some overseas French colony'.
But since the Eastern Europeans weren't really civilized, well documented reports of them being murdered in the millions by the Germans - made public before the half way mark of the war had even been reached - caused almost no outcry, beyond a few exiled or immigrant Poles and Jews....