Because it was the kind of war that was very debilitating. It was a war against civilians for one thing.
Poison gas which could maim as often as kill, left a person with permanently damaged lungs.
The war was fought in trenches. Plenty of opportunity to murder or be murdered, very little opportunity for clever espionage at the front. There was, however, plenty of opportunity behind the lines, but "All Quite ..." takes place at the front.
