Where were the forces of Austria, Russia, Sweden, Spain, and Portugal located? How ready were they to carry on a war with Napoleon, should the campaign in the low countries go badly for the coalition?
In comparison, what about France?
Most of the other nations involved in the wars against Napoleon had their armies left in their native lands, Alexander I also felt left out of the final defeat of Napoleon since he liked to play the "Savior of Europe" card. As a result, the British contigent in Belgium and the Prussian contigent near Brussles were the only forces on the field ready to fight France.
However, France was able to turn around real quickly to be prepared to fight. The borders of France were already being guarded by Brune to Toulon, Suchet was watching the French Alps, and Rapp was in Strasbourg. A reason for France's quick turn around has to do with how the Restored Bourbons acted. They had little respect for meritocracy, handing out Legion of Honor medals to anyone with the cash (as was common in the Ancien Regime) and didn't do much to win over the army. With Napoleon back, the Army generally went to him as he was always a supporter of merit over birth.
If you have anymore questions, please ask. And remember that on the 18th of June, I will be leading a group AMA on the darkest of days, the 20th anniversary of Waterloo.
Edit: I confused Augereau with Brune and fixed the locations of the commands of the Prussians in Belgium and of Brune, also added Rapp.