"Only Shakyamuni Buddha fulfills the three functions of sovereign, teacher, and parent."

The Buddha is the sovereign of the human and heavenly realms and the parent of all living beings. Moreover, he is the teacher who leads and opens the way. Though one may be a parent, if of humble station, one cannot at the same time assume the role of sovereign. And though one may be a sovereign, if not also a parent, one may inspire fear. And though one may be both a parent and a sovereign, one cannot be a teacher as well.

The various Buddhas [other than Shakyamuni], since they are known as World-Honored Ones, may be regarded as sovereigns. But since they do not make their appearance in this sahā world, they are not teachers. Nor do they declare that “the living beings in it [the threefold world] are all my children.” Thus Shakyamuni Buddha p.344alone fulfills the three functions of sovereign, teacher, and parent. - On Prayer