Why Is The Word "Retard" Considered Offensive When The Word Emerged To Replace Earlier Labels?

The word "retard" began to be used to describe the "mentally challenged" in the late 19th century to replace words like "idiot", "imbecile" and "moron" all of which had started being used as insults and had originally been used as medical labels for different low IQ ranges. Considering that "retard" was invented for the purpose of not being offensive and all it meant(before being used as a euphemism) is "to slow or impede" suggesting the individual is just "slow" how is it considered offensive now? Also, does the history of the word mean that people who claim it is offensive (and that people who use it are being ignorant) are actually the ones who are ignorant as they are clearly ignorant of the word's history (or they would not consider it offensive)?