Battle Art had always been monopolized by the nobles. It had never been passed down to anyone outside of the noble family or to commoners because the core of Battle Art was only passed down to those related by blood. Any commoners who wanted to acquire Battle Art had just one way, and that was to join the army and then climb up the ranks to become knighted. The knight would then be rewarded with Battle Art from the noble family he served.
But this was a treacherous path because it was almost impossible for a commoner to become a knight. They would have to fight tooth and nail for it, and nine out of ten who tried ended up dying. Even if they did obtain the Battle Art, it would only be the most basic level; the essential parts would be omitted.