Answer:
Explanation:
If you look carefully at the diagram, it looks like the French have no way out. The Prussians are on the outside perimeter and can always get supplies into their position. Since they are completely surrounded, the French do not enjoy that luxury.
Furthermore (and I'm guessing here) the Prussians probably outnumbered the French. Now it is possible to get around that problem, but not terribly likely if the numbers are really lopsided.