Moron, Idiot, and Imbecile were coined as precise medical terms by Dr Henry H Goddard in the 1900-10s. Retard(ed) was devised as a euphemism to replace them in the 1960s.
Once an unfashionable euphemism falls out of use, people gradually cease to recognise the offensive meaning which made it unfashionable.
Why?
(That you don't see any of them in official publications doesn't count for much in my opinion. Official publications try to avoid all negative emotive language.)