Why did he say so to Hashem?
Radak: How do You send me to them to speak via a Mashal? When I do so, e.g. the previous Meshalim, of the great Nesher (17:3) and "Imecha cha'Gefen b'Damecha" (19:10), they say?
Malbim: They say that I speak Meshalim and matters without explanation. They do not heed my words. The Rambam says that a condition of absolute Nevu'ah is that there are not in it Meshalim and riddles 1 . The multitude of Meshalim shows that imagination is overpowering, and the Nevu'ah is sporadic. Then he sees via windows and covers, like one who sees from afar.
Moreh ha'Nevuchim 2:45 lists 11 levels of Nevu'ah; the highest three do not have Meshalim. (PF)
Why did they say "Memashel Meshalim Hu"?
Rashi: I prophesized to them via parables and riddles, e.g. a forest, and a fire consuming moist and dry.
Radak: They say 'he speaks Meshalim, and it is not via Nevu'ah. He wants to show that he knows to give Meshalim. Therefore, Hashem repeated the matter clearly, without a Mashal.