It’s because China is selling itself as impartial. They are selling themselves as an alternative to the US who is well known for using sanctions against countries who displease them, countries who do things they don’t like, countries run by people they dislike.

China’s idea for peeling the world away from US hegemony is offering a better deal. To do that they feel a need to be consistent so that the US has absolutely no examples it can point to of them violating their policy of being impartial and neutral, of not conditioning trade ties on obeying Chinese edicts on domestic policy, on basically trading with everyone fairly and letting countries set their own policies and leaving it to be their internal business.

This kind of selling point is very attractive to many global south countries, some of which may be run by people who are oppressive, the gulf monarchies for example are a group China would like to at least pull closer to their corner.

The second they break this rule for any reason, people start asking questions about why they won’t break it for reason x or for country C doing bad things and it becomes a hammer to hit China with from two fronts: 1) That they like the US will try and use their might to influence your domestic policy, that you risk becoming a vassal to China like with the US if you side with them 2) they start getting hammered for doing it in this instance but not in another instance and the US does it in that instance and they start comparing themselves and China starts looking badly and can either buckle and risk significant real-politik consequences being lured into a trap of sanctioning whoever the US pleases or having to weather that storm.

This is a consistent position that places them beyond the morality of individual nations. They believe they need to build their strength first and for US hegemony to cease before they can even conceive of using power that way because any early use and they risk people fleeing from them. Let’s remember most nations are capitalist and already have a built-in wariness of siding with or being with a communist power like China over a capitalist one like the US.