The Zanpakutō of Gin Ichimaru, Sōsuke Aizen and Kaname Tōsen are not materialized by Muramasa because this anime-only arc takes place sometime after the events of the Soul Society arc, and as such the positions of 3rd, 5th and 9th Division Captains are absent. In short, Shinsō does not gain a human form in this arc plainly because its Shinigami master is not present.