Rashi says that Hashem set this up in order to honor Moshe. Once he set it up then it became possible for the Leviim to continue setting it up and taking it down. My son said at his son's bris (Yom shlishi Parshas Pekudei) that because of the kedusha of the mishkan, not even Moshe could (physically) put up the walls. Hashem had Moshe act and He caused the walls to be erected.
Once that was done, and the Chanukas Habayis completed, the kedusha of the mishkan had been established. Also, Moshe Rabbeinu had established that the mishkan was now completely set up. The Leviim had received their tasking and were now in the position of "working for Hashem". At this point, Hashem allowed them to carry out their appointed tasks and to succeed in taking down and setting up the mishkan.
This means that the reason that the Leviim could not set it up originally was a decree from Hashem as was the neis of Moshe (appearing) to set it up by himself. Once Moshe had completed his task, Hashem decreed that the Leviim would be able to perform their assigned tasks.
This is another way of teaching the lesson that a person does his hishtadlus (effort) but it is actually Hashem who causes the effort to succeed or fail.
We can see a similar situation with the Aron, which "bore its carriers"
Now they brought the Mishkan to Moses, etc.: Because they could not
erect it. Since Moses had done no work in the Mishkan, the Holy One,
blessed is He, left for him the task of erecting it [the Mishkan],
since no human being could erect it [by himself] because of the
heaviness of the planks; and no human was strong enough to put them
up, but Moses [was able to] put it up. Moses said before the Holy One,
blessed is He, “How is it possible for a human being to erect it [the
Mishkan]?” He [God] replied, “You work with your hand.” He [Moses]
appeared to be erecting it, and it arose by itself. This is [the
meaning of] what it says: “the Mishkan was set up” (Exod. 40:17). It
was set up by itself. [This is found in] the midrash of Rabbi
Tanchuma. -[from Midrash Tanchuma 11]
Our rav, on Shabbas Parshas Pekudei, cited Rav Yaakov Galinsky who calculated the weight of the kerashim. It was almost 4 metric tonnes per board. This is based on 58 cm. per amah. .5 grams per cubic cc of wood (atzai shitim is considered a light wood). The plated gold is estimated at 8mm thick at 19.5 grams per cubic centimeter. Thus the boards had to be put up by a neis.
Each board was 10 X 1.5 X 1 Amah = 2,926,680 cubic centimeters
.5 grams per cubic centimeter = 1,463,340 grams = 1,463.340 kilos = 3,219.348 pounds
gold covering (on inside and outside) (9 x 58) (excluding the pegs) x (1.5 x 58) x .8 cm x 2 (two sets of covering) = 72,662.4 cubic centimeters
19.5 grams per cubic centimeter x 72,662.4 = 1,416,916.8 grams = 1,416.9168 kilos = 3,117.21696 pounds