1. Bnei Yisrael was not receptive to Moshe's message that the redemption was immanent. Moshe Rabeinu went back to Hashem and argued (6:12) that his mission was futile: If Bnei Yisrael won't listen, kal v'chomer then certainly Pharoah won't listen either.
Everybody is troubled by this kal v'chomer. Bnei Yisrael didn't listen because they had no time to think and listen; they were being beaten and persecuted day in and day out. Why does the fact that they didn't get the message mean that Pharoah won't get it either? (Previous post on the torah of the Noam Elimelech on this.)
The Sefas Emes answers that the road to Pharoah's ears was through Bnei Yisrael's hearts. Pharaoh was not the real cause of Bnei Yisrael being freed or enslaved. As we see later in the parsha, Pharoah eventually lost even his bechira -- he was not in control of his own destiny or that of Bnei Yisrael. However, so long as Pharoah appeared to be in charge, so long as Bnei Yisrael believed that he controlled their destiny, there could not yet be a full geulah. Geulah means recognizing ain od milvado -- there is nothing other than ratzon Hashem. All the obstacles to freedom fall away once that one truth is accepted.
Hence Moshe's argument -- if Bnei Yisrael are not yet ready to accept that message, all the rest is moot and nothing will help.
2. Taking a step back, Hashem certainly knew that Bnei Yisrael wouldn't listen. Why then did he send Moshe only to be rebuffed?
Turning to the Sefas Emes again, he explains that this parsha was written to teach us that even when people don't listen -- even when the listener's mind is closed and words of Torah seem to bounce off a hardened shell of indifference -- those words still leave an impression. There is no such thing as wasted chizuk and hisorerus.
(A nice Sefas Emes to keep on your pulpit shtender to peek at when the you feel only the walls are listening : )