Because after almost 25 years of war (1791-1815), France was exhausted. Decades of continuous warfare had taken severe tole upon the French population (as defined by those who lived within pre-1792 borders) and, despite being well led and often achieving favorable casualty ratios, suffered over 1.7 million casualties. This, as percentage of military age males, was possibly -more- devastating than WWI a century later.

This problem of manpower had already manifested itself earlier, and Napoleon resolved it by recruiting heavily from Non-French populations in the French Empire and client states. Paul Kennedy estimated that the Grand Armee which invaded Russia was composed only between 25%-33% of Frenchmen from pre-1792 France. Which meant that the VAST MAJORITY of the army were composed of foreigners: Poles, Italians, Dutch, Germans etc. This source of manpower was largely inaccessible to Napoleon come 1815.

So when 1815 comes around, not only are the enemies of France much more organized and powerful than in the 1790s, but France itself had being bled white in Napoleon's wars, and thus even if political will had existed for it, is unlikely to be able to institute the levee en masse that the First Republic had.

That oddly is the thing, he was in the process of raising troops and those that were with him were soldiers that came to Paris or followed him from his landing.

When he arrived in Paris, he called for the raising of a hundred thousand soldiers but since he was defeated within a couple of months, Marshal Davout (acting as War Minister) couldn't get everyone together. Further there were other armies to feed, such as the Army of Italy and the Army of the Pyrannes. So the situation was very much going to be like that of the early Revolution.

The people were tired of war but they didn't respond well enough to Louis XVIII and would rather have Napoleon.