From France's pov, it all started as a way to centralize and wrest power away from the provinces, later as a way put France and French at the center of Europe. Having been the center of science and education for much of the middle ages, renaissance, colonial power, etc. It's hard for France to give in symbolically and allow that there are stronger influences than its own.

It's been a quixotic quest, at best, it's not as if in regular speech (French) people didn't inject whatever was the argot dujour into the speech be it domestic or foreign.