Why did Hashem answer the question?
Nedarim 81a: No one knew to give a reason, except for Hashem.
Why does it say "Azvam Es Torasi"?
Nedarim 81a: They did not bless on Torah beforehand 1 .
Malbim: There are three levels in guarding Torah. (a) One clings to Hashem's Torah with love and wondrous desire. Corresponding to this, it says "Asher Nasati Lifneihem" (that it should be their souls' delight). (b) He guards it because Hashem commanded, and he fears His punishment. Corresponding to this, it says "v'Lo Sham'u v'Koli." (c) He does according to habit - "Mitzvas Anashim Melumadah" (Yeshayah 29:13). Corresponding to this, it says "v'Lo Halchu Vah." In all these matters, they chose other gods.
Ran (Nedarim 81a, citing R. Yonah): If they abandoned Torah and did not engage in it, like the simple reading of the verse, this would be known. Chachamim and Nevi'im would have explained it! Rather, they engaged in Torah constantly. Hashem knows the depths of the heart, that they did not bless on Torah beforehand. I.e. the Torah was not so important in their eyes that it is worthy to bless on it. They were not engaging in it Lishmah, and amidst this they belittled its Berachah.
Why does it say "Nasati Lifneihem"?
Radak: "Lo va'Shamayim Hi...; v'Lo me'Ever la'Yam Hi" (Devarim 30:12-13).
