Yes, the real reason for the vanishing of the Moabites, Ammonites and Philistines is that God’s judgment was against them. To his prophet Ezekiel, Jehovah declared:

I will make Rabbah a pasture for camels, and the villages of the Ammonites a resting place for flocks. . . .

There is overwhelming evidence that this prophecy was fulfilled. Ammon and Moab did indeed become the “possession of the Easterners.” Says archaeologist G. M. Landes:

“Archaeological explorations have shown that sedentary occupation in the land of Ammon virtually ceased before the middle of the 6th century B.C. . . . Into the vacuum created by the destructive advances of the Babylonians poured once again . . . the ‘sons of the East,’ the bedouin hordes from the desert, who now destroyed all formally organized political activity in this area, and brought to an end the semi-autonomous Ammonite state.”​—The Biblical Archaeologist Reader, Vol. II, pp. 87, 88.