Why Avraham's offspring need Eretz Yisrael

ברש"י כאן אי אתה זוכה לבנים פי' כי בודאי הבורא ית' לא יפלא ממנו כל דבר אבל לא הי' בכח המקום של חו"ל לקבל קדושת זרע אברהם.

In Rashi here: "Here [outside the Land] you will not merit children" — the explanation: for surely nothing is too wondrous (impossible) for the Creator, may He be blessed; rather, the place of chutz la'aretz (outside the Land) did not have the capacity to receive the holiness of Avraham's offspring.

The Sefas Emes clarifies Rashi: Avraham's childlessness in chutz la'aretz was not because Hashem was unable — nothing is beyond Him. The limitation lay in the land itself, which lacked the spiritual capacity to hold the holiness of the seed of Avraham.

ואם הי' בכח להוליד תולדות בחו"ל לא הי' גלות והי' מתוקן הכל.

And had it been possible to bring forth offspring in chutz la'aretz, there would have been no galus and everything would have been rectified.

If the lands outside Eretz Yisrael could have sustained Avraham's holy children, the whole concept of exile would never have been necessary, and the world would already stand fully repaired.

אך המקום לא זכה לזה:

But the place was not worthy of this.

Only the soil of Eretz Yisrael possesses the sanctity to receive Avraham's offspring; chutz la'aretz simply did not merit that capacity — which is why Avraham had to be brought to the Land.

Summary: Avraham's inability to have children outside Eretz Yisrael was not a limitation in Hashem, for nothing is impossible for Him; rather, the land of chutz la'aretz lacked the spiritual capacity to receive the holiness of Avraham's offspring. Had any place been able to hold that holiness, there would be no galus and the world would already be perfected — but only Eretz Yisrael is worthy of bearing the seed of Avraham.