Why does Moshiach have to be a descendent of King David?

Why must Moshiach be a descendant of King David? G-d promised that King David’s lineage would carry the eternal quality of royalty, with kingship embedded in their very essence. Only a true king with royal blood has the power to unite the Jewish people as one. Therefore, Moshiach must come from King David’s line to fulfill this divine promise of eternal leadership.