User blog comment:Kurare/Zero Division powers & inventions theory/@comment-25480434-20140929215152

These are a few reasons I think they were promoted to royal guard. Oetsu was the creator of the basic sword form of the asauchi (zanpaktou). My guess for senjumaru is that she was the creator of the uniform that soul reapers wear and probably the captain robes as well. I believe that tenjiro was given his place as a royal guard because of his insane healing techniques and the pond that boost peoples spirit energy. Which is pretty op if you ask me. Kirio was the creator of the gigai and of course her reiatsu infused food. Ichibei was given his place because he named everything from the zanpaktou to bankai.