?Vatashuvu Vativku lifnei Hashem?. What is the significance of the fact that they mentioned the Name 'Hashem', which is the Name that denotes Rachamim?
Rashi: Because they accused the Midas ha'Rachamim, of being insensitive to their cries, insinuating that Hashem was being cruel.
Why did Hashem not accept their Tefilos - despite the fact that 'the gates of tears are never shut?
Ramban and Seforno #1: Because Hashem had issued a decree with a Shevu'ah, and a decree that is accompanied by a Shevu'ah cannot be repealed. 1
Seforno #2: Because of the Chilul Hashem, for which only death can ultimately atone.
Refer to Bamidbar, 14:41:3:1-3 and notes.
See also Oznayim la'Torah.
Why did Moshe add "ve'Lo He'ezin aleichem"?