Because they were the offspring of Jacob’s brother. The Israelites were not to molest or war against the Moabites and the Ammonites, for they were descendants of Abraham’s nephew Lot.
كما امرهم ألّا يُعادوا او يثيروا حربا على الموآبيين والعمونيين لأنهم متحدرون من لوط ،‏ ابن اخي ابراهيم .‏