Niggers, coloreds, negroes, blacks, African-Americans, POC, BIPOC, it'll be something else soon enough.

I suspect 80960KA's assessment of the reason why is right on the money as well, you can see it happen with other words that shouldn't have any negative connotation, such as "Mexican".

See also "retarded", used to be a medical term, then it was the common vernacular, now it's a "slur". "Special" sorta became a slur too, even though they literally made it to be sensitive. It turns out being retarded is the thing that's bad, not the word itself.