The Rebbe reveals the answer by comparing the narrative of the spies sent by Moshe and the narrative of the spies sent by Yehoshua 40 years later, before entering the land to conquer Jericho.
From the Rebbe’s explanation:
– Among the differences between these two narratives are:
a) There was no direct command for Moshe to send spies. Rather, G‑d left the matter up to Moshe’s discretion as Rashi comments on the word לדעתך (‘according to your opinion’) in the opening verse of the Torah portion. In contrast, Yehoshua was explicitly commanded to send spies. This is obvious; after the disastrous results of the mission of the spies sent by Moshe, he surely would not have sent spies unless commanded to do so by G‑d.
b) In regard to the spies sent by Moshe, the Torah uses the expressions “men” and “explore.” In contrast, in regard to the spies sent by Yehoshua, “spies” and “search out,” expressions which reflect more clandestine activities, are used.
c) Moshe sent twelve spies and Yehoshua sent only two.
d) In regard to the spies sent by Moshe, the Torah mentions the names of the spies and specifically states that they were the leaders of the people. In contrast, the identity of the spies sent by Yehoshua is not mentioned in the narrative.
e) The spies sent by Moshe were sent openly; the entire Jewish people knew of their mission. Furthermore, there was no attempt to hide their mission from other nations. They traveled as a group, in a manner which their presence could be noticed by anyone. In contrast, Yehoshua “secretly sent spies,” hiding the matter from the Jewish people and surely, from the Canaanites.
f) The spies sent by Moshe traversed Eretz Yisrael in its entirety. In contrast, the spies sent by Yehoshua were instructed to “see the land and Jericho,” (at the outset, their mission had a more limited scope). Furthermore, in actuality, they merely went to Rachav’s house, fled to the hills for three days, and then returned to Yehoshua. Thus, they did not explore the land as a whole, and did not even explore Jericho in its totality…
To be continued…
Based on the Rebbe’s talk on Shabbat Parshat Shelach, 26th Day of Sivan, 5751 (1991). Free translation.