The spiritual leaders do not comprehend what is really going on, and neither are they leading the people in sorrow over that. As spiritual leaders they are to provide leadership to the people in serving God.
The priests have another special reason for leading the people in this display of mourning: after all, food and drink offerings will no longer be brought into the temple (see Joel 1:9), and this must first and foremost be of concern to the priests.1
13 Put on sackcloth and lament, O priests; wail, O ministers of the altar. Go in, pass the night in sackcloth, O ministers of my God! Because grain offering and drink offering are withheld from the house of your God.