In an age where radio was new, where televisions were even newer, where transcontinental news took hours, days or even weeks to travel, the knowledge that the Nazis were engaging in mass murder was not readily available.
When allied troops liberated the concentration camps they filmed everything that they saw because they were concerned that without footage no one would believe them. Why? Because it was a shock, a shock so unbelievable in writing that only video and photographic evidence would suffice.