Mainly because the victorious coalition powers forced it on France. In 1814 the Sixth Coalition had over a million men under arms, about half of whom were in France, and had directly occupied Paris, much of NE France, and SW France. Alexander did have the French Senate assemble. And he did treat with them. But they didn't actually do much in the way of negotiating or in establishing the new state. Instead after Napoleon's abdication the Coalition negotiated directly with the Bourbons rather than any of the old republican assemblies. As much as possible the Coalition tried to ignore the revolution. For example the ceasefire wasn't signed by any Republican or Imperial authority, it was signed by the Bourbon, Charles Count of Artois (future King Charles X).

That's not to say that French sensibilities were ignored. Before the restoration, Louis XVIII was made to write up a new constitution. Napoleonic law was largely kept in place, and few of the old privileges of nobility were restored. But for all that the Coalition was looking to appease the French, they didn't really negotiate with them. The Bourbons were forced on the French.