Others knew long before that revelation and were not given sufficient voice by a complicit media, not to mention how the Roosevelt administration acted towards Jews who were under Nazi control.
The full extent of the Nazis crimes were generally believed to be exaggerated till indisputable evidence became available towards the end of the war.
Partly this is because the crimes were so horrible it seemed reasonable that they couldn’t really be entirely true, and partly because war propaganda during WWI greatly exaggerated German atrocities to whip their population up for the fight.
Once burned, twice shy and all that.