I would speculate it has something to do with the 100km populaire being adopted as an alternative to the imperial century maybe during the 1970s in the USA?
Century road club manual from 1898 only lists imperial centuries and there seems to be a drop off in American interest in long distance cycling until the late 60s/early 70s. I bet if someone had the time/energy they could look at major contemporary published manuals and see when the term first popped up.
Glenn's Complete Bicycle Manual, 1973 is mentioned in the century ride Wiki as being a source for this quote:
A sanctioned century ride is organized and conducted under the rules and liability protection of a sanctioning organization, such as the League of American Bicyclists.[1]
So if one were really curious they could dig up that edition, see if it had a metric century as well and if not keep going forward until the term pops up.
Bikeforums own carpediem has a blog post mentioning the bloomin' metric century taking place in the early 1980s:
http://sprinterdellacasa.blogspot.co...ic-and-me.html