“Moshe spoke before Hashem, saying, ‘Behold, Bnei Yisrael did not listen to me, so how will Pharaoh listen to me, and I have damaged lips?!’” (6:12)
Moshe was blessed with everything a person could possibly have. This was so that Klal Yisroel would believe that he was the true messenger of Hashem.
The power of speech, he did not have. Because if he was a smooth talker, people would say that people believed Moshe only because of his gift of speech. He could talk anyone into anything. On the other hand, if people follow someone who cannot speak well, it is assumed that he has a following because of his truthfulness and sincerity. (Derashot Hara"n)