Seeing as the king is the human vicegerent of the Lord, it is the Lord’s prerogative to choose a king for Israel.1 He will choose someone from Bethlehem, a town which was not particularly prominent at the time. Likewise in the future, the Messiah will come from a town that has little prominence (cf. John 7:41–44).2
1 The LORD said to Samuel, “How long will you grieve over Saul, since I have rejected him from being king over Israel? Fill your horn with oil, and go. I will send you to Jesse the Bethlehemite, for I have provided for myself a king among his sons.”