We know that Israel received prophecies of a King. For example you have as early as Numbers 24:17 where Balaam prophesies, against his will, the coming of a King.
I see him, but not now; I behold him, but not near; A star shall come forth from Jacob, A scepter shall rise from Israel, And shall crush through the forehead of Moab, And tear down all the sons of Sheth.
When I read it, it sounds like a King who is primarily local to that area. That’s not surprising. A prophecy given to a prophet in a specific area and situation; maybe he is given what he needs to know. Prophecy, after all, is not merely To Know Things but for building up, encouragement, comfort (1 Cor 14:3) and a call to repentance (1 Cor 14:24-25).
Therefore, you discover constant expectations of a King explicitly mentioned like in 2 Sam 7 or Psalms 2 as God laughs while his King is installed on Mount Zion or Psalms 45 where a king gets married.
This wasn’t a change in the plans of God. It wasn’t like things had gone so bad that a King had to show up to clean house. After all, Deuteronomy 17, the very Law of God, has a built in section on how Israel is to choose their Kings and how those Kings are to act. Even if later history (as the book of Kings goes about showing) doesn’t work out according to blueprint, the fact is that the expectation of a King wasn’t a change: someone had to rule.
This goes back as far as Genesis 1. Adam was to function as a King over creation, ruling as God’s representative and with God’s prerogatives. Creation was given to him and his spouse to enjoy and to rule over (Gen 1:26). The fact Adam failed didn’t attest to the failure of The King Project, but it pointed to the expectation of a future King over all, like Adam, who would reign and, unlike Adam, not fail in the day of testing.
Therefore a King that is demarcated as glorious (Psalm 2:7-10), supreme (Psalm 89:27), who sits on the very throne of God (Rev 3:21) and reigns on the throne of David (Isaiah 9:7; Ezekiel 34) was a necessary expectation. There was no other way to put a fallen and rebellious creation back into the position it was supposed to be: under God’s righteous control.
Jesus was that expected King. He had to be king.