http://www.torah.org/learning/perceptions/5765/shemos.html...
And so began the Egyptian exile. It is hard to imagine that after 3500 years we are still in it, especially since we have not been in Egypt en masse for almost the same amount of time. But, as the Leshem teaches, the redemption from Egypt had not been complete; we lost 12,000,000 Jews in the Plague of Darkness, which eventually resulted in exile into Bavel 900 years later, the Greek exile, and finally, the Roman exile that we are still in.
As we said earlier, Egypt was a people, but Mitzrayim is a concept, and they just happened to overlap during Moshe Rabbeinu's time. The people may have been destroyed in the Ten Plagues and at the Red Sea, but the concept lives on until Moshiach comes and ends its reign of darkness. For, Mitzrayim means meitzer (Mem-Tzaddi-Raish = constriction) yumm (Yud-Mem = 50), an allusion to the constriction of G-d's light, often referred to as the Nun Sha'arei Binah (the FIFTY Gates of Understanding).
The Arizal explains that the ideal purpose for going down to Egypt in this week's parshah, was to be a light unto nations. When Moshe Rabbeinu came back to redeem us 209 years later, we were dangling for our spiritual lives from the forty-ninth level of spiritual impurity. What he should have found instead was the Egyptian people thriving spiritually on the fiftieth level of holiness, made possible by the Jewish people's return to Kesones Ohr (Clothing of Light), the state of Adam HaRishon before eating from the Tree of Knowledge of Good and Evil.
So, instead of remaining in Egypt the full 400 years that G-d had spoken to Avraham about, a number that alludes to a complete period of gestation and development, G-d had to step into history and arrest a process that was spiraling downward in the wrong direction. The plagues not only began to destroy the spiritually impaired Egyptian nation, but also to begin some sort of healing process for the Jewish people. That in and of itself was unable to cure the vast majority of Klal Yisroel. The impact of the Egyptian lifestyle had been too great to peel back, even after eight supernatural plagues for four-fifths of the Jewish nation.
So, even with the introduction of redemption into our experiential vocabulary, we never really left exile. Even after leaving Egypt with an exalted hand and entering Eretz Yisroel with a strong one, we have never stopped wandering, at least intellectually. There has always been some element of crooked thinking that has infiltrated many Jews and that eventually spread to enough of the nation to warrant the destruction of two temples and countless exiles throughout the ages.