Why is Birkas Ha-Mazon featured at this juncture? The Torah recalled and promised God's bounty in many previous parshiyos; why was the mitzvah of Birkas Ha-Mazon not presented during these earlier narratives?
The thrust of Parshas Eikev is to be inspired to adhere to the Torah by recalling and keeping alive in one's mind and heart all that Hashem did for B'nei Yisroel and our obligations to God, reflective of His eternal bris (covenant) with the nation. Rather than live in a vacuum, basing life decisions on current perceptions and ignoring the past, the Jew is instructed to have a mindset and to act in accordance with Hashem's historical mastery of all, His redemption and care of our people, and His involvement with the world and supervision of all that occurs. The parsha teaches us to be instilled with appreciation for past redemptive miracles and God's sustenance and constant beneficence. Conditioning one's mind and heart to relate and react to all through this lens is the key to adherence to Torah for the future.
In this vein, the question posed above is readily answered. Birkas Ha-Mazon is not merely a beracha of thanksgiving; rather, it is a proclamation of God's mastery and sustenance, through which the Jew declares his eternal faith that God provides, sustains, redeems and supervises. Note that the first paragraph of Birkas Ha-Mazon is a statement of God's sustenance; the third paragraph is a testament to His past and future salvation; the fourth (Rabbinically-mandated) paragraph is an expression of recognition for His enduring goodness; the concept of thanksgiving only appears in the second paragraph. Birkas Ha-Mazon is an encapsulation of the theme of Parshas Eikev, such that one who recites it affirms Hashem's historical bris and our commitment to Him; it is for this reason that Birkas Ha-Mazon appears in Parshas Eikev. Birkas Ha-Mazon is a declaration of the message of our parsha.
Living in modern times poses many challenges, and we are obligated to condition ourselves to maintain a constant mindset of appreciating and living by God's covenant with us. Our perspective must be defined by Parshas Eikev, such that the message of Birkas Ha-Mazon reflects how we live, think and act.