Yoshimune became the eighth shogun even though the highest ranked gosanke was the Owari family, and the reason is generally explained that because his age was the closest to Ieyasu among the members of the Tokugawa gosanke.
徳川御三家の中から家康との世代的な近さを理由に、御三家筆頭の尾張家を抑えて第8代将軍に就任した、と一般的には説明されている。