From what I understand, Ousen is at his best when the battle comes down to the chaos of engagements where being able to manage that complexity allows him to come out on top even against stronger opponents, but Riboku basically cut off any attempts Ousen could’ve to make this battle chaotic, sending forces like the Ri Shin out of the picture who are known for their chaotic interferences into his strategies.
He basically took out all the unknown variables from the match until all that was left was just head on engagement, where his now very moralized forces would come out on top, and Ouse had no more cards to play.