If you mean why didn't God just free them since he is omnipotent this is because he wanted humans to have free will and that he would not interfere with the people or countries on Earth. If you are then asking then why did he do the 10 plagues in Egypt. This is because Moses was given a choice by God to free them and he did it in his own will and the plagues were not affecting the free will they were signs to Ramses (Pharaoh of Egypt) that God was real and to let his people go.
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Moses was the most righteous man (Numbers ch.12, Deuteronomy ch.34) and therefore the best choice to convey the Torah to the Israelites. See also the Related Link.
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