They are testing the waters of Pharaoh's with a much smaller thing, a three day prayer fest in the wilderness. Pharaoh's decision is not only based upon the request of the Hebrews, but, his image and perception of their God. In a sense he is absolutely right to ask who is this god and why should I obey him.
So the first approach did not work, in fact things got worse. The people blame Moses and Moses blames the Lord. It was actually a pretty lame approach to begin with, but, one has to realize that you don't just go up to a pharaoh and ask him to let my people go. Things have to be proven, leaders have to be tested, effort and sacrifice invested, God has to be depended upon.
2012 - pBiblx2 Field Wise Bible System Version 2.0.9d - GPL3