I hate it that words become unacceptable just over time and we all have to adapt our language. What could be more reasonable than describing someone as retarde

I hate it that words become unacceptable just over time and we all have to adapt our language. What could be more reasonable than describing someone as retarded? The word means "delayed or held back in terms of progress or development". No one would argue with that as a non-derogatory and accurate label, so why has the word fallen from grace?