The Children of Israel were given several mitzvot at the time they entered the land, so the Gemara focuses on their experience just before this time. For example, how close to the land of Israel were they when they stopped eating the manna? It's not as obviously "40 years in the wilderness" as one might think. Plus, tracking the date via Moshe's death on the 7th of Adar. Plus, how it is known that he was born on the 7th of Adar too, and the righteous implications of that. Also, more on the mitzvot that were to be practiced in the land of Israel, and some select few that apply outside of the land too - specifically with regard to debts and relinquishing debts.