Migration narratives in ancient Near Eastern Studies (Topoi E-CSG-V)

How assyriologists and archaeologists described the appearance of several ethnically defined groups or “peoples” in the “Fertile Crescent”? Why did migrations advance to crucial factors of explanation in the historiography of the Near East at the turn of the 20th century? And what was the reason for presenting certain migrating peoples like the “Semites” as destructive invaders whereas others like the Sumerians appeared as founders of early civilization?