The foundational distinction these sources draw is that outside the Land, divine vitality reaches the world only indirectly — a spark of "the word of God" illuminates the angelic ministers of the seventy nations from above in a surrounding (makif) mode, without actually clothing itself within them, and only from those ministers does life-force flow downward to the nations and to the physical earth and heavens (Tanya, Iggeret HaKodesh 25) — whereas Eretz Yisrael, under the direct, constant gaze of God described in Devarim 11:12, is the place where that higher unity can be drawn down through the deeds of those below (Torah Ohr, Lech Lecha 10c).
Because the Land receives divine attention directly rather than through intermediary forces, Noam Elimelekh (Bamidbar, Sh'lach) teaches that the Land affords greater power for elevating the holy sparks (nitzotzot kedoshot), and that this was precisely God's purpose in bringing Israel there — a sanctity so real that even the footsteps of the patriarchs left a spiritual imprint that made the work of elevation easier for subsequent generations.
Kedushat Levi (Bereishit, Lech Lecha) adds a further dimension: since each of a person's 248 limbs draws its vitality from the corresponding mitzvah, and since many mitzvot are bound specifically to the Land, Avraham could not achieve complete divine service outside of it — the Land-dependent commandments were missing, leaving whole "limbs" without their spiritual life-force, so that full embodied worship of the Creator became possible only upon entering Eretz Yisrael.