I've found this very strange as well.
Best guess is that it hasn't fully reached their populations / news audiences. It's only in the last four to six months that Western media has begun to actively talk about it and pursue the truth of what's going on there.
Alternatively they don't protest China because they know it does no good.
The Guantanamo Bay detention camp had less than a thousand total detainees over its history (currently an estimated 40 people held there), most of whom were pulled directly off of battlefields. Guantanamo became a global scandal and blackeye for the US, drawing countless mass protests across the Middle East.
Here we're talking about millions of Muslims being tortured, 're-educated,' stripped of their Islamic beliefs, and apparently many are dying or being murdered in the process. An estimated 1,300 camps and growing. All without any external access or oversight, happening under an aggressive rights-denying dictatorship.
These millions of Muslims are not just going to give up their beliefs willingly, so what is China's next step? Vast genocide has to be the concern going forward.