Why did the King of kings and Lord of lords have to come from David's royal family?
Because God promised David in 2 Samuel 7 that his house, kingdom, and throne would be established forever, and that from David's royal line would come the King of kings and Lord of lords. Christ came through David and Solomon's royal bloodlines, with Joseph as legal father, fulfilling that promise.
Les explains that in 2 Samuel chapter 7 David is promised that coming from his loins would be a royal house, or a royal family of kings who would rule and reign over Israel. Through that line of kings would one day come the King of kings, Christ, born of Mary with Joseph as His legal father. Both Mary and Joseph come down the royal family bloodlines of David and Solomon. God's promise was that David's house and kingdom would be established ever before him, and his throne forever. Les emphasizes that God doesn't lie and cannot lie, so this royal family would go down through history and beyond history into eternity. Out of David's royal family would come the King of kings and Lord of lords, and He will come as He did the first time to the nation of Israel, which is why they have to be in the Middle East at the time of His coming.