He could not mention the real reason.
He was Destiny Monk, and everyone, including the King, worshiped him like a God. Based on that, he could not just tell them straightforwardly that his array formation was broken through by Gu Chaoyan, who made almost no efforts.
His master left the array formation for him so that he could defend himself. And that was the only array formation he was able to make.
Since the array formation was broken through and the secret revealed, he was definitely going to be considered as someone incompetent. The King was a very skeptical person, who would even start to suspect his real identity.
He had the mission of dealing with the Phoenix Girl, so if the King lost his trust towards him, things would get worse.
Destiny Monk moved no inch. He kept thinking about a proper explanation.