Well, the answer is surprisingly personal, as in, in large part because Klemens von Metternich personally wanted it that way as part of maintaining the pre-Revolutionary balance of power because it was valuable if not essential to absolutist rule, and the European powers who had led the Sixth Coalition were more happy than not with his vision of European peace because the conservative victors of Waterloo who weren’t Austrian were 1. at their most liberal still only British constitutional monarchists who A. were still themselves aristocratic landed gentry and didn’t care for too much democracy or bourgeois reform at home B. primarily cared about isolationism and wanted stalemate through balancef power on the continent because a balanced Europe prevented Britain from being dragged into wars or 2. just honest-to-god absolutists like Prussia and Russia worried about French-inspired populist revolution at home, given the events of the last 20 years. Metternich chaired the Congress of Vienna on behalf of Austria and was an aggressively, repressively anti-liberal who saw the necessity of cooperation among Europe’s monarchies to each maintain their individual dynasties in the face of bourgeois liberal revolution, or even liberal reform that would chip away at ancient (and proper) absolutist/monarchical rights to govern. (There are some who blame the First World War on Metternich because his maneuvering at the Vienna Congress and afterward to maintain Austrian absolutism and generally hold back the tide of liberal reform allowed or at least helped Austria to continue as a cultural patchwork rather than reforming along cultural, nationalist lines, nationalism being a co-product of bourgeois ideas of self-determination, self-governance, and ‘the state’, and it was the desire for Bosnian independence that led to Archduke Ferdinand’s assassination, which we all remember launched Europe’s web of defensive pacts).

To explain Metternich’s thinking, not in a divine-right-to-rule manner (which, I understand he believed in), but pragmatically, the simplest version is: a strong France was necessary and good to keep a strong Prussia in check, and a strong Prussia was necessary and good to keep a strong Russia in check, and a strong Russia was needed for their antics around the Caspian to keep the Ottoman Empire busy across two fronts to prevent them from entirely focusing on Austria’s possessions in the Balkans. France’s territory had to go somewhere if not to France, and that necessarily meant either strengthening someone else, probably Prussia or Austria, or engaging in the kind of artificial state/dynasty creation along the lines of the French Empire’s sister republics, and that had a slew of problems that were just harder to manage - the new dynasty would lack legitimacy and need support from other absolutists, and aside from still moving the fulcrum on which the entire mess of European monarchical regimes rested on, Metternich guessed (basically exactly correctly) that unless the threat were existential to the whole system of aristocratic government, as revolutionary and then Napoleonic France had been, most of the conservative governments (except Tsar Alexander of Russia) were pretty happy to abandon each other to fend for themselves - cf the Belgian Revolution that split the United Kingdom of the Netherlands. Some rulers might’ve seen a shift in power as an opportunity for further Austrian gains, but history pretty well showed Metternich was correct in guessing that this particular genie of republican/populist/nationalist government and bourgeois reform/rule was particularly dangerous to hereditary privilege, would not go back in its bottle, and would only be kept away with active repression and cooperation among aristocratic regimes, so anything which damaged the legitimacy of absolutist rule in France would also lend itself to liberal/bourgeois revolution somewhere else on the continent, so rather than open that Pandora’s box, it was safest to just... turn the clock back, and kind of pretend the whole thing, ie the last two decades, hadn’t happened, which included letting France remain a strong opponent to its victors with its borders, in addition to the ill-fated Bourbon Restoration.

In Great Power negotiations in the fate of nations, we always have to ask (1) what Powers benefit from partitioning a weaker power (2) which Powers dominate the decision-making (in this case the Congress of Vienna), and (3) what norms and geopolitical imperatives do diplomats carry into the room?

Geopolitical Goals matter: Many of the rank and file, as well as the upper officer corps, did have nationalist and vengeful goals that rhyme with those of a later nationalist era. But they didn't dominate or dictate policy. Europe in 1815 had aristocratic / monarchic leaders. The monarchic Coalitions' goals during the French Revolutionary and Napoleonic Wars was not to crush France, but to restore borders ante bellum and stabilize France by placing its monarchy ante revolution back. The Coalition was not Britain's show. Much of the Coalitions' leaders were traditional allies of France. They wanted France back as an ally.

In 1815, the European Great Powers were monarchies, and all save Prussia officially and at least superficially, truly believed in restoring the "Balance of Power" as their primary geopolitical goal. Destroying France, the largest demographic nation in Europe, was fundamentally incompatible with this goal. Humbling France was fine, but France was already humbled. I personally believe that what people say often determines what people want. By all accounts, Tsar Alexander wanted to restore the peace of Europe by restoring the 'legitimate' monarchy to France. By all accounts, Metternich despised nationalism and wanted to tamp it back down the bottle. Giving the French dynasty back its crown while taking land to fulfill German or Belgian nationalist goals was seen as both petty and counterproductive.

Neither Britain nor Russia wanted partition. The Congress of Vienna is strangely enough, often seen as the Austrian Minister Metternich's show (largely because as head of the locale, he dictated procedures), but it very much was not Austria's show. Austria had rebounded from the wars to an even greater extent than Prussia, but it was still firmly second-tier to Russia, which had largely restored it. Russia and Britain both wanted stability in their respective spheres, which they dominated, which were also and importantly, different areas of interest. Britain had humbled France's navy and thus had safeguarded its American and Asian colonies. Britain's elite wanted, as they always do, to retreat back to their normal status of salutary neglect of the Continent while economically exploiting the colonies. Similarly, Russia had gained hegemonic sway on the Continent, and thus could keep its main goal of continued domination in north Eurasia, with France to counterbalance Prussia and Austria. Destroying an old ally like France and enabling a new land grab by potential rivals was not seen by Russians as enabling the restoration of long-term peace and stability. For all of Metternich or Tsar Alexander's weaknesses, they were aiming for the long-term, per their own discourse.

Obviously some parties wanted partition. Savoy. Netherlands. The German kingdoms and duchies. They weren't running the show. Partitions are a funny thing. On the one hand, it is tempting for a stronger Great Power to take what it can. On the other hand, it is difficult for several Great Powers to compromise on how much each power gets. The Partition of Poland was very much a fairly rare anomaly. Opportunity is key, and 1812/1815 saw the absence of one strong Great Power with a desire and ability to partition France. Prussia and Austria alike were prepared to partition parts of France, but their own rivalry with one another hobbled a united front. Neither Tsar Alexander nor the British Ministry wanted to see France crushed geographically. They did not stand to benefit, so they consistently stood in the way of any partition that would strengthen a united Imperial Germany at the expense of France.