They do not like a particular term that may have negative connotations. So, they promote a new term without those negative connotations, a euphemism.
Some current euphemisms applied to those with impaired mental abilities :
Retarded (not advanced?)
Special (not usual?)
Disadvantaged
Challenged
Me, I'm "vertically challenged". Not my fault, so I don't feel any shame about it.
I'm short, there is nothing that I can do about it, so I'll live my life being CHALLENGED to get items off the top shelf.
I am DISADVANTAGED because I have to find a step stool, or one of those neat little mechanical grabbers.
I'm SPECIAL, because I'm the one who really needs to remember where the step stool is kept.
And I am RETARDED, because everyone else is out the front door ahead of me. They are ADVANCED, if you will, because I am still in the kitchen putting away the step stool.
I am called a SENIOR citizen. Actually, my dad was Sr., I'm Jr. Me, I'm just getting OLD.
The problem with euphemisms is that they, too, eventually acquire negative connotations and must be replaced with yet another euphemism.
Euphemisms prevent us from talking about uncomfortable subjects. Sometimes these are things that should be talked about and understood.
In 1979, there was a secret meeting in the basement of the WH in which president Carter decided that every term used to apply to groups of Americans or certain characteristics would be deemed to be “offensive.” Thereafter, a secret commission went around the country complaining about the old terms and urging protest to change the terminology. It was conceived as a joke, but it was so secretly funded that remnants of it exist today.