most redditors are from the USA, and the USA generally has a really bad history education.

my own US history class didn't even teach WW2 because it was too busy jacking off to the american revolution, and my world history class spent 80% of its time on the european renaissance. if you'd asked me at graduation to name some global human rights atrocities, the only thing i could tell you: was US chattel slavery, and the german holocaust (which i only learned about in english lit class...yes really.)

so yeah, this is a big part of the reason some "obvious" events of world history are big WTF moments of revelation for some of us. we quite honestly had no idea, and we can't go seeking out knowledge for a specific even if we didn't even know the event happened.