Why? Besides Tiananmen, China has the same government and leadership culture that perpetrated the Cultural Revolution. Going back slightly further to the 1800s, the Taiping Rebellion killed 20-70 million people in China, humanity's bloodiest rebellion/Civil War. Chinese culture has a looooong history of violent repression of internal dissent IMO....
But if your baseline expectations are low, you are rarely disappointed. For an organization or individual to have its expectations changed from "low" to "high", there should be some basis for such an upgrade, primarily actions taken by the organization that demonstrate higher performance (for whatever metric you are using to define "performance", in this case protection of human rights).
Does the Chinese Communist Party have a demonstrable history of embracing or advancing human rights? IMO, no. So they remain at the "low expectations" baseline.
Censorship is not an excuse for their behavior. It's a further criticism of the Chinese state. The suppression of ideas and history is something which makes outcomes like these more likely.