Radak: It was to avoid being Metamei the land, even though they are not Yisraelim. "V'Lo Setamei Es Admasecha" (Devarim 21:23) - an unburied Mes is Tum'as ha'Aretz. Also in the future, Yisrael will bury Gog and Magog "l'Ma'an Taher Es ha'Aretz" (Yechezkel 39:12).