I think it has more to do with the success of the language standardization effort.

AFAIK being very serious about that was a declared goal historically; nation states in general not that old etc, etc..

So you have lots of people that went through a purposefully rigid process in education and of course with that through greater society. At the end there is maybe not much room / tolerance in handling deviations from the norm?

Despite all of that I’ve also seen this changing though as France / the world has become less insular (learning more foreign languages, etc).