Rashi does not usually explicitly write the driving question in his commentary. In this case, the question is obvious: why did Moshe call an assembly to himself at all? Moshe already declared that "I can no longer go out and come in," which Rashi explained as: "Permission has been taken from me and given to Joshua." Considering that there is only "one leader per generation and not two leaders," why was the assembly (which belongs to the leader of the generation) done by Moshe and not by Joshua?
To explain this, Rashi first investigates the difference between Moshe and Yehoshua’s authority. Since the authority of the trumpets had not been transferred to Yehoshua, this indicates that certain aspects of leadership, symbolized by the trumpets, never were given to Yehoshua. This explains why Moshe called the assembly and not Yehoshua, because Moshe still held onto certain leadership privileges until the end of his life.
Once that is settled, then Rashi explains why Moshe himself did not use the trumpets to call the assembly, because they “were hidden away even during his lifetime, prior to the day of his passing...and there is no ruling on the day of death.”
