A lot of people have a fairly superficial understanding of ML theory, so they see China having capitalism as a betrayal of the core principles. Effectively this shows lack of class analysis on their part. My reply to this point is that if China allowing a limited form of capitalism makes it capitalist, then Canada must be communist because we have some social services like free healthcare. It's an absurd line of argument to make, yet it's the level of understanding a lot of people are stuck at.

Once people internalize the idea that China is communist only nominally, then other stuff easily builds on top of that. For example, the atrocity propaganda becomes easy to swallow because obviously capitalists would exploit minorities, we see them do it in our own countries and everywhere around the globe.

I suspect another big aspect is chauvinism. People just don't want to accept that China managed to succeed where their society failed. It's much more comfortable to believe that China is just a different branding of the same system they live under. Acknowledging that China is a successful socialist society run by a principled Communist party means also having to grapple with the failure of western left.