Apparently they don't like being called retarded anymore. Groups have been self-advocating to get it stricken from medical usage. Personally I think this is retarded. Even a guy in the article basically says that any word for a person of lower intelligence is going to be used derogatorily.
http://www.washingtonpost.com/wp-dyn/content/article/2010/02/04/AR2010020402602.html