Why did German forces live off the French countryside during the Franco-German War?

During the Franco-German War, German forces were compelled to live off the French countryside because they outran their supply lines.1

At the time, the Prussian army utilized the Etappen system, which relied on a chain of magazines and shuttling wagon trains to move supplies from railheads to the troops. This system was designed for a slow, steady advance along a rigid schedule. However, because the German forces advanced more rapidly than their horse-drawn wagon trains could keep up, the supply system failed to maintain the necessary pace. Consequently, the troops were forced to rely on local resources in France, which was one of the richest agricultural regions in Europe, to sustain their momentum.1