Why did Moshe fall on his face?
Rashi: He fell on his face out of helplessness
See Sifsei Chachamim. Rashi: By the Eigel (Shemos 32:1), by the Mis'on'nim, Beha'aloscha, 11:2 and by the Meraglim, Bamidbar 14:13, he Davened, but when ut came to the current, fourth sin, he felt powerless, because 'How many times can one intercede with Hashem on behalf of Yisrael?' (See Rashi).
See Ramban (Ibid. DH 've'Hinei').
See Ba'al ha'Turim.
Why did only Moshe fall on his face and not Aharon?