Perhaps one of the constraints against coining such a word has to do with the inadequacy of the English languge to express in a single word the status of the surviving parents. It is clear that "orphan" defines a person who has lost both parents; and it is clear that "widow/widower" defines a woman/man who has lost her/his husband/wife. But how would one define parents who have lost a child but who may (or may not)have other surviving children?