The job of a King is to lead his people in a fashion that his people will not sin to begin with. The job of a High Priest is to help the people repent and achieve atonement after they sin.

Because, the High Priest teaches to people how to Love, G-d, and love allows for the person to feel the intensity of their ego I. The King teaches his people Fear, which breeds self-abnegation.

Hence, "Fortunate is the generation, whose leader takes heart to bring an atonement for even his unintentional sin."