Just throwing my tuppence in here. Religious garb, cassocks, nun's habits (pre Vatican II) and Academic robes, were all the normal wear of persons in the middle ages. As fashion changed, they stood still, pretty much so anyway. Since the traditional academic wear came from Northern Europe, it is signally unsuited to wear in much of the US in June, the traditional month of Graduation.

I had a colleague who, in his attempt to stay cool(er) had trousers legs with elastic tops he wore around his lower legs, and a shirt-dickey thing that had just enough showing plus a collar for his tie.

I will go along with the idea of horses being the cause of the demise of the unbifurcated garment, makes a lot of sense to me.

Remember, fabric, before the days of powered spinners and looms, was extremely expensive. Hence the practice of stripping the garments from the condemned and dividing them among the executioners. The same held true with the dead on the battlefield, they were routinely stripped and the "spoils of war" divided amongst the victors.