As entertaining the messengers of the emperor or the retired emperor cost a huge amount of money, the Edo bakufu appointed only tozama daimyo as Kyooyaku to prevent them from saving extra money.
勅使と院使の饗応には莫大な予算がかかることから、幕府は余計な蓄財をさせない意味で外様大名ばかりを任命した。