A new form of government emerged with Moses. During 400 years of slavery in Egypt, Israel has developed into a nation. God chose Moses to lead the children of Israel out of Egypt into the promised land. This would not be easy, because the Egyptians did not want to lose their slaves, the Canaanites did not want to lose their land and the nations in between were afraid of the nation on the move. Moses was primarily a military leader with responsibility for getting the people into the new land. He had been trained as a leader in Egypt, but after a false start he escaped to wilderness, where he was called by God. The calling was clear.
The LORD said, "I have indeed seen the misery of my people in Egypt. I have heard them crying out because of their slave drivers, and I am concerned about their suffering. So I have come down to rescue them from the hand of the Egyptians and to bring them up out of that land into a good and spacious land, a land flowing with milk and honey... So now, go. I am sending you to Pharaoh to bring my people the Israelites out of Egypt" (Ex 3:7-10).
No successor was appointed for Joshua (Jos 23). The reason is that the task that Moses hard started was now completed. Israel no longer needed a military leader. God had promised that if Israel obeyed him, he would keep them safe from their enemies (Deut 28:1-7). The Moses/Joshua model was not the optimal form of government. It was a temporarily leadership model for a unique situation, which has not occurred again.