If it is only an exception in Chinese history, why is it not being mentioned anywhere, let alone discussed? You still don't see the difference?
Most Chinese don't want to talk about it because it's considered a source of national shame. (Westerners often interpret that as them being afraid to talk, but that's not actually true.) They also believe the events are used for foreign propaganda which is not entirely untrue.
In Germany people have a completely different perspective on the Holocaust. It was horrible and a source of great national shame, but still, never forget. Blocking Wikipedia on Holocaust remembrance day would not happen in Germany.
Is the Hundred Flowers Campaign also an exception? And the millions of people who were killed during the Great Leap Forward and Cultural Revolution? Or the millions of people who have been brainwashed for religious reasons in Xinjiang? Or the thousands of journalists, authors and publishers who have been jailed, sometimes "disappeared", for texts critical of the CCP just during the last two decades?
I guess there was the brutality of the Tiananmen Square Massacre, but Tiananmen is the exception in the history of Chinese protests, just like the Kent State Massacre was in the US.