For most of their history both Italy and Germany were split into various independent republics and monarchies. Yet when unification finally came in the latter half of the 1800s, Germany became a federal state, while Italy became a unitary state. Why?

I can offer some insight from the Italian perspective, and can point you towards this answer of mine from a few years back where I touch upon some ideas which might interest you.

In the aforementioned answer, I argued that a key difference is that unification in Germany was peaceful and unification in Italy was violent. This does not mean that I believe german unification was entirely peaceful (individual cases like Hannover certainly stand out, and wars vital to the process did happen between Prussia and other powers) nor that Italian unification was entirely violent (local activists and leaders were often willing to welcome the Piedmontese, whose armies marched unobstructed through much of Italy). In short, what I mean is that while German unification was punctuated by conflict, but wasn't defined by it; on the other hand Italian Unification was very much defined by conflict and war.

Italian unification was originally a deeply populist movement, where "Unity" and "Nationalism" meant different things to different people. Only when unrest on the peninsula became too widespread to ignore was it co-opted by the political leadership in the Kingdom of Piedmont-Sardinia (which I will call Piedmont for brevity) where the "spin" on unity developed by the liberal-bourgeois political majority would define unification as the creation of a large liberal state on the peninsula. Telling is the fact that soon after the Kingdom of Italy was proclaimed in 1861 a four-year legislative furor saw significant reform and redesign of civil, commercial, and administrative legal codes, with the intent to actualize the political leadership's liberal vision. Unification, so the liberals believed, would have hardly been worth it if a federal system had allowed the Kingdom of Two Sicilies or Papal States (just to cite the two most reactionary and conservative regimes on the peninsula) to keep their regressive social systems which not only impeded the nationwide economies of scale necessary for an industrial economy, but could continue to instigate popular revolt (or rather, allow unrest to turn into the the "Wrong Kind" of popular revolt that might demand deeper changes than the Piedmontese were aiming to implement).

France’s direct influence on the Piedmontese, and more generally Italian, political preferences is also not to be discounted. While I won’t deny the Napoleonic experience had an analogous effect in Germany (or maybe it didn’t, I’m no expert on Germany) the proximity of Piedmont to France, and not to mention the Piedmontese experience of having been directly annexed by France, as well as Napoleon’s partiality to Italian affairs (he did crown himself “King of Italy” after all) ensured that while the Piedmont’s ruling House of Savoy did not recall the Napoleonic experience fondly, the bourgeois intellectuals who populated the cabinet and legislature instead very much did. And while Germany would be proclaimed in 1871 before a defeated Second French Empire in the Palace of Versailles, Italy would instead be proclaimed a decade earlier in Florence on the back of significant french military and material aid. Thus it was easy for the Savoyard monarch to look at the very centralized French system with begrudging admiration: it had first dethroned his house, and now made him King of Italy. As Piedmontese soldiers marched across the major squares of Italy’s conquered capitals, the Italian political class very much knew this was the model they would like to follow when modeling their newly founded country.

Could there have been parts of Italy which might have been granted more autonomy within the Italian system, perhaps as a reward for spontaneous requests of annexation? Probably not: I will insist that the process of Italian unification was defined by conflict, and even those Italian States which willingly offered themselves to Piedmont did so with the language of war, with letters of unconditional surrender and offers of dictatorial powers to the Piedmontese monarch. Every one of these annexations occurred when Piedmontese forces were tied up with the difficult task of confronting the mighty Austrian Empire, and thus in the wake of even the most peaceful annexation the immediate preoccupation of the Piedmontese leadership was to neutralize Austrian-backed institutions (almost the whole of Italy apart from Piedmont) as well as to pacify the unrest which had led to Austrian-affiliated ruling dynasties to be outright expelled, local armed forces to disband, and local institutions to evaporate, and which now risked to grow and spread unbridled should they fail in their efforts to achieve rapid annexation. Centralization, and the imposition of the Piedmontese system from the top-down, was seen as the natural consequences of the brief (and however purely formal) Piedmontese occupation that occurred immediately after rival institutions on the peninsula utterly collapsed before the mere specter of war and local unrest.

This collapse, which allowed the Piedmontese to so quickly rebuild institutions from the top-down, was also determined by fact that outside of Piedmont and the territories directly held by Austria, administrative systems weren’t particularly well-developed to begin with (contrasting with Germany) and indeed the prospect of joining a sprawling french-style bureaucracy was very appealing for bourgeois Italians who were one of the groups most ardently in favor of unification. The reactionary states of Italy had also pretty much expelled all their loudest intellectuals to Piedmont, meaning that all the Italian states' most politically active citizens were was already well-integrated within the political system which the Piedmontese would impose onto them. So there just wasn’t an appetite within the political-intellectual class, either in Piedmont or in the conquered Italian states, to preserve the institutions which had existed prior to unification as in a Federal system: The Italian states were just too strongly perceived as ineffective puppets of the Austrian domination on the peninsula, which completely collapsed before revolters emboldened by the mere specter of the Piedmontese advance, and if they weren't replaced, risked spreading "the wrong kind" of unrest across the peninsula. There wasn’t anything political decision-makers were willing or even able to save, and so they didn’t save anything.