John Calvin Commentary Deuteronomy 17:15

"thou shalt surely set him king over thee, whom Jehovah thy God shall choose: one from among thy brethren shalt thou set king over thee; thou mayest not put a foreigner over thee, who is not thy brother." — Deuteronomy 17:15 (ASV)

1. You shall certainly set him king over you. God maintains His own supremacy in the appointment of a king and does not entrust the matter to the people’s own votes; so that He might discipline their audacity in demanding a king according to a hasty impulse.
2. He commands that the king should be taken from the people themselves and excludes foreigners, because, if foreigners had been admitted, a door would have been opened to apostasy; for each would have tried to impose his native gods upon them, and true religion would have been persecuted by the force and threats of the royal power.

This is why God would not allow a king to be sought from anywhere else but from the midst of His Church; so that he might cherish and maintain that pure worship which he had learned from his childhood.