The Torah tells us that upon the completion of the building process, the Jewish people brought the parts of the Mishkan to Moshe Rabbeinu (שמות ל"ט:ל"ג-מ"א), and that he then blessed them for their efforts (פסוק מ"ג שם). Later, the Torah says that Moshe actually did more than merely accept what they brought him and bless the people for what they had done; he actually erected the structure himself, as it says "המשכן... ויקם משה את," "and Moshe erected the Mishkan" (מ':י"ח שם). Rashi (שם ל"ט:ל"ג) explains why Moshe had to build the Mishkan himself, indicating that the people could not erect the Mishkan by themselves, and that because Moshe himself had not done any of the work for the Mishkan, Hashem left it to him to erect. When Moshe asked how it would be possible for him to do this by himself, Hashem said that he should make it appear as though he is erecting it, but that it ultimately will stand up by itself. No one person, including Moshe, could possibly have erected the beams of the Mishkan by himself because of their weight. Moshe was able to do so only with the miraculous intervention of Hashem. The question can be asked, however, as to why it had to be erected by only one person in the first place. During the travels of Bnai Yisrael in the desert, there is no doubt that many men worked together to assemble and dismantle the Mishkan. Why could it not have been done the same way the first time it went up? Why was it so important for Moshe to appear to have been doing it himself?
HaRav Yosef Dov Soloveitchik often spoke about the concept of כנסת ישראל. By this phrase, he meant more than the idea of the Jewish people, connoting the plurality of Jewish individuals. כנסת ישראל refers to a singular entity, as if it is one individual which can not be broken into parts. All Jews together make up כנסת ישראל, an idea which has important Halachic ramifications regarding ownership of communal sacrifices, Eretz Yisrael and so on. The only individual who could represent this idea of כנסת ישראל was Moshe Rabbeinu himself. He was not only the leader, but he also embodied the essence of the people. He was the heart of the nation. Moshe had to be the only one to raise the Mishkan to a standing position because by doing so, he showed the very meaning of it. The Mishkan did not belong to any individual Jew, nor could any Jew claim his piece of it any time he wished. The Mishkan belonged to כנסת ישראל, and an individual Jew could claim it as his own only to the extent that he felt at one with the Jewish nation. Thus Moshe, and Moshe alone, as the embodiment of כנסת ישראל, had to put up the Mishkan himself that first time.