I thought that had originated in the le de France and changed French before it changed German.
I can’t either.
But that’s because I don’t know enough about languages to make a difference
If I were to redefine the grammatical terms for Swahili, I’d call it “totems” rather than “genders”. 
It is no wonder less inflections may “catch on” so well, for most people don’t wish to continue overfilling their mouths with excess syllables on the ends of words when they may shave off most of such syllables and achieve saying basically the same thing, with a few adjustments.
But something such as “of” instead of a genitive in many places, is probably more just from less inflections starting to feel more “natural” to the speaker, and therefore, even such a simple inflection may become used less and less not because the genitive is not important but because the manner of using “of” now feels more “natural”.
What made me always wonder, this is not why inflections and genders die. To me it seems all natural that a language tries to get simplified and yet retain its full expressions, though in other forms, much easier to use. But why were all these complicated rules of grammar at the beginning created? It looks the older a language is, the more complicated the rules are. Today, we can live without any dual gender, so why couldn’t they? Perhaps someone has an answer.
Happy new year to everyone!
Some “sine quibus non” linguistic readings about language change (Historical-evolutional linguistics)…
GRAMMATICALISATION!!!
Google —> “grammaticalisation”
You really have to read : Hopper, Paul J., and Traugott, Elizabeth C. (1993) Grammaticalization. Cambridge University Press.
And the most usefull: Barry J. Blake: Case !!!
Östen Dahl, The Growth and Maintenance of Linguistic Complexity
It isn’t a one way street; if it was Latin and Greek would already have lost their case systems 2,000+ years ago. Romanian is an example of a modern language which lost the case system of its ancestor language, but then developed a new one.