[RickarySanchez]

The stigmatization is imagined at best. Its similar to how words to describe individuals with learning disabilities also gain a negative connotation. Its not the word its the fact that the word refers to a subset of people that a comparison to is an insult. Hence, Mongoloid -> Retard -> Special -> <whatever the new one is>

In terms of racism, its different but the same mechanism. Being compared to a minority race is not an insult (to most people). Its the fact, that racist people will use the word with vitriol. Racists and those they argue with will use the term in their arguments and gradually the use of the term will gain the conotation of a racist person. Hence, Negro -> Colored -> Person of Color -> <the next thing when PoC becomes racist>

[OtherShrezzing]

I think _Mongoloid_ doesn't fit with the rest of your examples, as it was never originally an impartial term. There's never a time when _Mongoloid_ wasn't an offensive term to some group, whereas _retard_ & _special_ were originally impartial.

[ZeroGravitas]

With terms for learning disabilities (and some racist terms) factual inaccuracy is also a factor.

Mongoloid, Retard, Special, individuals with learning disabilities are not entirely interchangeable (except as insults).