This may be because Moab tended to be less violent than other nations of that time and because there was a connection between Israel and Moab. The Moabites were descendants of Lot (Genesis 19:30–38), and in Deuteronomy 2:9, the Israelites are told not to engage in battle over land with the Moabites. In addition, Ruth, who later became the great-grandmother of David, was also a Moabite (Ruth 1–4).
9 Therefore I weep with the weeping of Jazer for the vine of Sibmah; I drench you with my tears, O Heshbon and Elealeh; for over your summer fruit and your harvest the shout has ceased.