Rashi gives the most quoted reason, which is that Bnei Yisroel were so intent on collecting the wealth that washed up with the dead Egyptians that they had to be forced to leave. Rabbi Sorotzkin in Oznayim LaTorah gives two other reasons that actually tie into modern politics. The Egyptian army had been destroyed. Egypt was no longer a world power. Indeed, we do not hear of the Egyptians again for almost 500 years, when King Solomon married the daughter of the resurgent Egyptian Pharaoh. This is an illustration of the point that most people lose sight of the long term goal because of the short term results that face them.
As a result, there was a group of Israelites who wanted to settle the area of the Yam Suf (Reed Sea) where they had emerged. Since they no longer feared the Egyptians and G0d had shown his power at that spot, they fell for the then current superstition that a god had his main power at certain locations in the world. The closer to the center of power that a people remained, the better the god would be able to show his power. The ten plagues were actually a prelude to the use of power and it was here at the sea that the center of power had been manifest.
Another group of Israelites wanted to go back to Egypt and tak over the government. Moses would become the Pharaoh and the Children of Israel would become the owners and their former owners, the Egyptians, would now be their slaves. This is indeed like most of the revolutions that have occurred in the world (except for the American Revolution).The former "oppressed class" instead of embracing freedom and learning how to behave, merely take the place of their former oppressors. This is similar to the results of the French and Russian revolutions. It is also what actually happened when the empires of Europe left their African colonies.The former slaves had no model of a society other than what they had seen from their masters. Since they had been slaves, they would never have seen the actual methods of governance or had a chance to learn what was actually required to build a society.
This is why the American Revolution was actually able to succeed. The people had built a society for themselves already and were defending "the rights of Englishmen" rather than uprooting what had been imposed on them. Similarly, the Bnei Yisroel needed to be taken to Sinai and given the laws of the Torah in order to be shown the society that was going to be built.This is also one of the reasons that they needed to have an entire generation grow up under the rule of the Torah before they could enter Eretz Yisroel and attempt to live on their own.
