The mitzvah of saying birchas hamazon has to do with thanking Hashem for our food. What does entering Eretz Yisrael have to do with it? We had to eat even before we got to Eretz Yisrael. Why should the bracha have changed then and why davka is that part of the bentching the most beloved bracha?
The Sefas Emes explains:
As a result of the sin of Adam the earth was cursed, “arura ha’adamah…,” and man was told that he would have to eat food grown by the sweat of his brow. “B’itzavon tochalna” – eating was never the same again.
We have a rule that “ain arur misdabek b’baruch,” something cursed and something blessed are like oil and water – they just don’t go together and can’t combine. Hashem wants to bring bracha into the world, but a cursed earth needs to be redeemed first.
The first blessing of birchas ha’mazon was said over the man, food that fell from the sky, not food that was grown from the ground. It’s no wonder that Moshe and Klal Yisrael were able to say brachos over it.
The far greater chiddush is that a bracha can be recited over plain old bread and fruit. Eretz Yisrael is described in our parsha as “eretz tovah,” a good land. It is a land where the curse of “arura ha’adamah” can be overcome, where food untainted by the cheit of adam ha’rishon and the punishment of “b’itzavon tochalna” can be grown. Bnei Yisrael's conquest was not just a battle of physical might, but it was a spiritual transformation of the land known as Eretz Canaan, land which belonged to the grandson Canaan which Noach cursed, into Eretz Yisrael, a land which could attach itself to bracha.