Because Chinese capacity was recognized early under Obama, who "pushed" first with pivot to Asia. There was acknowledgement that China could potential competitor, sick Olympics 2008 followed by Chinese growth bailing world out of during GFC same year. Incidentally this was when China first realized their own power levels / brokenness of US system. This was under Hu. First Obama term 2009, Secretary of State Hilary Clinton recognize that China as US most important relationship of the new century, that Chinese raise need not be treated as an "adversary" - implicitly acknowledging that China as potential future competitor. This was followed by US increasing and strengthening Asia-Pacific military deployment forging alliances, selling arms to Taiwan to box in China behind first island chain. Xi inherited this problem in 2013. The response - South China Sea build up in 2015. BTW China was the last party out of 6 claimants to build land and militarize the region. It was reactionary. Similar to how US pushed NATO to undermine Russian peripheries, so byebye Ukraine. Ditto with SCS. Then you have TPP and technological competition after announcement of Made in China 2025 which was met by containment efforts on tech front.

Queue Trump + Bannon + bolton + Pompeo (all Chinahawks): tradewar -> huawei -> hk -> xinjiang -> covid -> tawian -> tech coldwar.

Reality is, US has been trying to contain China for over a decade now, it's merely very public and acrimonious under Trump admin because that's his style.

I don't know if politburo thinks China's time has come, but never waste a good crisis. And it seems like US is committed to full blown containment, and the cost benefit analysis now of HK as financial instrument is superseded by HK being exploited to undermine the Chinese state. There's realist reasons to act sooner than later, HK protests have killed 1C2S and peaceful reunion with Taiwan already, at least for current generation. But reunion deadline is still 29 years away. A lot can happen during that time.