The gemara (Baba Basra 75b) writes that Yehoshua was a pale comparison to Moshe – “Pnei Moshe k’pnei chamah u’pnei Yehoshua k’pnei levana.” Moshe was like the sun, Yehoshua like the moon. The gemara continues that when the Elders saw this they bemoaned, “Oy lanu m’oso busha,” Woe to us from such embarrassment. Why were the Elders in particular affected by this difference between Moshe and Yehoshua? Would not all of the people feel the loss of Moshe Rabeinu's leadership?
The Mishna (Rosh haShana 2:10) tells us that the names of the members of Moshe’s beis din are deliberately omitted from the Torah so that if a person feels that the judges of his era are incompetent, not worth listening to, we can always argue that they are no worse than the anonymous members of Moshe’s court – there is no way to prove otherwise. The Divrei Shaul quotes the Ketzos haChoshen as explaining that this is why the Elders in particular were so shocked when they saw the discrepancy between Moshe and Yehoshua. The Elders realized that the dramatic difference between Moshe and Yehoshua indicated a trend of continuing precipitous decline in leadership over time. And yet, theoretically, some future judge, obviously on an even lower level, could be compared to one of them!
The Ksav Sofer offers another beautiful explanation. Yehoshua was the leader of a new generation, a generation that did not see Yetziyas Mitzrayim first hand and who did not witness Moshe Rabeinu in his prime. That generation saw that Yehoshua was not on the level of Moshe, but they assumed this was to be expected given Yehoshua’s relative younger age and lack of experience. However, the Elders had seen Moshe when he was the same age as Yehoshua;, they had seen Moshe when he was also new to leadership. They knew that it was not age or experience alone that could account for the difference between Moshe and Yehoshua, and therefore it was these Elders who felt the full brunt of the decline.