My own suspicion is that it's a matter of fashion. More specifically, there are lots of problems in the world, and someone is always going to be insulted when one of them is referenced lightly. Right now it's fashionable to treat the word "retard" as a politically incorrect slur, so those particular people have social permission to complain about it.

What I can't make up my mind about is whether or to what extent we should approve or disapprove of the use of these words.