Bediuzzaman Said Nursi answers this question in his book “The Letters,” in The Risale-i Nur Collection:

Through His munificence, in order to gratify the Noble Prophet (Upon whom be blessings and peace's sentiments), Almighty God did not put His Noble Beloved’s parents under any obligation to him. His mercy required that to make them happy and to please His Noble Beloved, He did not take them from the rank of parenthood and put them in that of spiritual offspring; He did not place his parents and grandfather among his community. However, He bestowed on them the merit, virtues, and happiness of his community. Indeed, if an exalted field marshal's father, who has the rank of captain, entered his presence, he would be overwhelmed by two opposing emotions. So, compassionately, the king does not post the father to the retinue of his elevated lieutenant, the field marshal.