One could ask why the US and european countries are allies. I think it is because both are technologically advanced, both are rich, both are societies with human rights, both are democratic. (Relatively speaking, at least.) These things can change. Now that I name four factors it may sounds like a lot would need to change. But the first two as well as the second two are closely related. And at some point the former two are also related to the latter two. If China wants to keeps developing it will at some point need to improve its human rights situation as well.