User blog comment:CoolJazzman/Why do you think Yamamoto wasn't giving a promoted to become a royal guard./@comment-2120712-20130418162659

I think creating the Gotei 13 and the academy qualified him for promotion but he probably declined and who would run these programs with him gone? They organizations needed him more than the RG needed him. He has made significant strides in increasing shinigami proficiency but they are so large that he needs to stay and watch over them.

As Aizen said, he is the History of the SS or G13 but again he is needed more by them. Plus he created organizations while the other RG created or capitalized on certain theories and objects.