The request of Bnei Gad and Reuven positions them outside of the Promised Land, outside of God's plan for the historical future of His people, and even outside of the heritage of Avraham. Moshe’s counter-offer depends on the the loyalty Bnei Gad and Reuven promise their brothers, as well as loyalty to the land of their brothers, and to God.

Avraham
Lot
Egypt
Eastern Bank of the Jordan
Eretz Yisrael
Reuven Gad and Menasheh
Sodom