But there is also the cultural distinction as well: the invaders were generally herdsmen, not farmers.
They would want to keep the farmers right where they were, producing - and just make sure that the rent and obeisance went to them instead of the rulers they displaced.
The notion of "total war" is about as new as the notion of "equality."
Wars were fought, for the most part, while the common peasants sat and watched.
The Third Punic War came close to the modern definition of "total war," and was effectively a genocide against the Carthaginians. There are several Biblical examples of total war as well.
“Wales was the primary Celtic holdout in Western Britannia during the ascendance of the Anglo-Saxons. Using computer analysis, the researchers explored how such a pattern could have arisen and concluded that a massive replacement of the native fourth-century male Britons had taken place.
http://harvardmagazine.com/2009/07/who-killed-the-men-england