A few thoughts with no foundation, but wasn't kimono the main form of wear back then? Not very practical on horseback, I can see how hakama could be introduced for horse riding. I don't think there was much in the way of cow and sheep herding going on back then either, so maybe there wasn't really a need to make them out of anything but cotton. How about the leather industry, land is a premium due to the vast quantity of mountains?
Someone above mentioned sumo wrestlers, they still wear kimono today. I used to live next to a very famous sumo dojo in Tokyo. It was quite common to see the wrestlers in full kimon stocking up on snacks in the local 7/11.
For waht it's worth.