One theory is that the initial imperial court consent was approved on the basis that the Tokugawa clan was of Fujiwara clan (rather than Minamoto clan) stock however, when Ieyasu set up the Edo bakufu (Japanese feudal government headed by a shogun) the convention was that a Minamoto clan member be the Shogunate's leading unifying Shogun (commander-in-chief).
一説には、当初朝廷の許可では徳川氏は源氏ではなく藤原氏の氏族として認められていたが、家康が江戸幕府を開くに当たって、幕府の長たる征夷大将軍は源氏という慣例があった。