User blog comment:CoolJazzman/Why do you think Yamamoto wasn't giving a promoted to become a royal guard./@comment-6342763-20130420222438/@comment-7658432-20130424091820

2 things. First: All members of the Royal Guard were originally from Yamamoto's 1st batch of Captains. Supporting this is the fact that the most recent members was Kirio who was the 12th Division Captain during her time.

Second: The first organization was the Royal Guard, Yamamoto is a member of it and he just stepped down because he created Gotei 13.

Though the first is more likely correct.