Reports about what's happening in concentration camps had been coming to Allied intelligence agencies since 1941 ([1]) but obviously they did not include any video footage, so they were hard to believe.
In hindsight being 20/20 it's hard to comprehend the blurred optics of the past era.
Moreover, the blood libel of WW1 was still a fresh memory, and so many who did hear about Nazi atrocities were skeptical.
The world would be different today if not for the photographic and video evidence.