Because Poutine got bad advice from Xi. Poutine (wrongly) believed that Xi understand the West.
I noticed, there were not many news from Xi for a very long time.
He has 20th party congress looming, and he will not rock the boat.
Even if Russians will get railroaded, or even if Putin himself end up in the soup, Xi is unlikely to suffer, unless he does something stupid Ukraine related which will cause a blowback back at home.
Whatever camp in Beijing will come on top of Ukraine war result, they will still be weaker than the "3rd term camp"
Xi's camp message became very simple - "3rd term for XI, and that's it." They don't make bets on anything else, so only their previous mistakes can be used to mount assault on them.
Xi will side with the winner camp, but whack them after. Policy doesn't follow slogans in BJ. I.E: 18th congress "economic liberals" have "won," their policies are adopted, but they themselves now eat steam buns in Qincheng prison.
There is certainly some isolationist sentiment now, but again, it's inconsequential to coming internal events. China may turn more isolationist if Russia will be railroaded, and less if they will win, but overall internal direction is decoupled from that.