Mentally retarded as a phrase is no more "technically right" than "moron", "idiot", or "imbecile". All medical terms that preceded "retarded" for describing intellectual disabilities. You just like it because it happens to be the term that was being used at the time you learned it. It doesn't make it any more right or wrong. There is a medical term for the concept, its starts to become widely and popularly used as an insult, the medical profession comes up with a new term.
> Up until around the 1960s, the terms "moron", "idiot", "cretin" and "imbecile" were all genuine, non-offensive terms to refer to people with mental intellectual disabilities and low intelligence. These words were discontinued in that form when concerns arose that they had developed negative meanings, with "retard" and "retarded" replacing them
a) society, for its own protection, needs a term for "Person you shouldn't trust alone in a room with a small child"
b) nobody ever wants to be identified, personally, with that term, whatever that term is
It could be due to some filter feeling that it is inappropriate to use “retarded” as an insult, because it is disrespectful to people with actual mental disabilities.
I am around forty, but twice in the last couple of years a person half my age has taken great offense to me using the term “mentally retarded” to describe people who are actually, well, mentally retarded. So, I suspect that the euphemism treadmill is gradually consigning “retarded” to obsolete vocabulary in any context, and those who continue to use it may be seen as horrible people.