God rejected Saul as the King of Israel because he disobeyed God in the war against the Amalekites. In 1 Samuel 15, God commanded Saul to destroy the Amalekites and everything that belonged to them, but Saul spared the life of their king, Agag, and kept some of the spoil for himself and his soldiers. This act of disobedience was a clear violation of God's command, and it showed that Saul was not fully committed to following God's will. As a result, God rejected Saul as the king of Israel and chose David as his successor.