Because Auschwitz-II took dehumanisation to a whole extra level of depravity, far beyond slavery.

While you could logically argue that people there were also slaves, it would be an injustice to leave it at that. Generally speaking, slaves had some kinds of rights that were far better than those in Auschwitz-II.

It's one thing to be treated as someone's property and forced labour, as a slave.

It's another thing to be subject to awful medical experimentation (often to death), torture of kinds that are barely imaginable, and mass killing with on-site factories literally built to murder the occupants and other occupants forced to dispose of the body parts of their friends afterwards. All of this in large numbers.

There's a reason Auschwitz-II had sections called "Crematoria I-IV".

Auschwitz-II makes almost every evil in the world look mild in comparison. It's that bad.

I'm just trying to get the comparison of terms down. While, yes, as an American, English is my first and primary language, I'm afraid some of its phraseology still confuses me. I think it has to do with parts of society that I don't understand, granted. The visceral reaction to the word "slavery" seems much more than to the more sanitized phrase "forced labor", or even "labor camp". Those are what we are what we're discussing here, like it or not. Uighurs are being sterilized and used as forced labor, but the phraseology is so sanitary as to not disturb our modern pallet of consumerism, we dare not draw a comparison to Nazis. While there are no crematoria, there are certainly shallow graves and executions, and parallels to be drawn. The word slave and forced labor is, at best, an argument of semantics of note only to those on the sidelines I'm sure, while those in bondage don't care what they're called.