Board Thread:Episode Discussions/@comment-25319479-20160407114523/@comment-25319479-20160408224141

Zabuza became a member of the Seven Ninja Swordsmen before his attempt assassination on Yagura.

So if Juzu, who is in possession of the Kubikiribocho kills off Yagura, Zabuza can't get a opportunity to assassinate him while being a member of the Seven Ninja Swordsmen, since Yagura's already dead.