Because by the time naruto shows up theres 3 people left, currently in the animetheres four
fuguki,mangetsu gari and pakura. as we know fuguki goes down when naruto shows up, pakura and gari are down when kakashi leaves and we hear tehres still one swordsmen standing thats gonna be mangetsus episode
Mangetsu was never sealed this early he is sealed later on in the war. He is to much of a boss. I hope suigetsu get's to this level because right now he seems no wear near.