Imagine if every few years the Gregorian calendar had two Decembers. When would people shop? We of course are faced with a similar dilemma seven out of every nineteen years, as our calendar has two Adars. While we celebrate Purim in Adar II, this was actually the subject of rabbinic debate.
Raising an independent nation is no easy task. The generation that left Egypt was (understandably) not up to the task. Whether or not their children would be was something of an open question. Moshe spends almost the entire book of Devarim preparing the people so that their entry into the land is a successful and permanent one. He was undoubtedly worried about what might be; he knew he would not be joining them. The last time Moshe left B'nai Yisrael, they built a golden calf, and much of Sefer Devarim is devoted to the theme of the dangers and enticements of idolatry.