Even though it’s post-Norman conquest, the language was still evolving. Heck, it’s still evolving today. The Brits continued to influence the language through their communications with both (what is now) England and the lower French dialects.
But early Middle English still retained a good deal of morphological complexity, and that was hundreds of years later. German aside, most of the other languages in its family have also lost much of their grammatical gender systems, and Danish in particular is in some aspects even simpler in its morphology than English (although it does possess more irregular verbs).
Lawyers, man. Making society more complicated from day one :p. But I wholeheartedly agree : reading The Accursed Kings blew my mind re. the intricacies of the feudal society. It was much removed from the “trebuchet diplomacy” version I had been taught in school.
I stand corrected after a bit of research : the English kings did have to pay homage to the French ones as I remembered, but over their possessions in France (Anjou, Aquitaine and Normandy) rather than oversea. And that’s what the Edward the Third eventually refused to do (or rather, he was OK with paying his respects to Charles VI through a weak homage oath, but wouldn’t be his liege).
As with most conflicts, I doubt one could point to just the one “real” reason.