I enjoy reading histories that focus on the geographical factors that influence economic, intellectual, political, and military developments, etc. At the most surface level it seems that the same geography that allowed the Romans to conquer the Italian peninsula might have facilitated subsequent unification efforts prior to the 19th century. Either that thinking overemphasizes the role of Italian geography in the Roman conquest, or other factors trumped it. I'm wondering what explanations historians have considered.
As always, I thank the historians who post here. This is the most educational subteddit I've found, and one of the most entertaining.
The thing is, Italy was unified; after Odoacer discarded the title "Imperator" in favor of "Rex Italiae," Italy continued to be a largely Roman-style kingdom up to the second half of the tenth century. The political fragmentation in Italy following the tenth century is, in part, a consequence of it's own wealth, urbanization, and agricultural levels. These same factors strengthened it's lack of unity following the growth of trans-atlantic trade, leaving Mediterranean trade comparatively stagnant; as the Italian States stagnated, all they were able to do was maintain the status quo.
Let's begin at the beginning. The year is 476. The Western Roman Empire has lost Gaul (to the Franks) Iberia (to the Visigoths) and Africa (to the Vandals). Only Italy and Dalmatia are ruled from Rome. Years of civil war and infighting have led to long succession of army-backed strongmen taking ruling the empire, and in 475, the Magister Militum (head of the army) named Flavius Orestes propped up his son Romulus Augustulus while Julius Nepos, his deposed predecessor, set up a court-in-exile in Dalmatia.
Flavius Odoacer, another general, deposed Romulus Augustulus once he lost the army's favor. However, Odoacer openly admitted that he was powerless beyond the Alps, and consequentially didn't call himself Emperor. Rather, he sent the imperial paraphernalia to Constantinople and asked to be recognized as "Consul", keeping all of the functions of the Roman administration in Italy intact. Odoacer initially played lip service to Exiled emperor Julius Nepos, minting coinage in his name, but may have been involved in the plot whereby he was murdered. Regardless, Odoacer eventually asked to be given the title of Patrician by the Eastern Emperor and be recognized as the ruler of Italy under the wing of a single Emperor in Costantinople.
So here's our first observation: while Spain France, and later England develop specific ethnic heartland (Frankish, Gothic, and Anglo-Saxon respectively) from which proto-nations can emerge, Italy never departs from the vague concept of "Roman-Ness" which was a legal identity, more than an ethnic one.
Now, around the year 490, the Eastern Emperor Zeno had his issues to deal with involving two Generals both confusingly named Theodoric (this in addition to social, religious, and political unrest in his own half of the empire). He takes action, convincing them and part of the army loyal to them to go conquer Italy. This way, they would stop annoying him, and he could also get rid of Odoacer, another potential opponent.
This "conqueror" of Italy, Theodoric the Amal, was technically King of the Ostrogoths but he had been raised in Constantinople. His people were largely Romanized: having lived within the Eastern Empire's borders for centuries (heck, you could even argue "Ostrogoth" meant "Inhabitant of Pannonia garrisoning fortresses on the Danube). So when Theodoric defeated Odoacer and set up the first Kingdom of Italy, he did his utmost to further maintain the Roman institutions, law, and judicial system. However, he did not rule from Rome, instead he ruled from Ravenna, seaside city conveniently defended by swampy surroundings.
Although Theodoric was de jure a vassal of the Eastern Roman Emperor, de facto the Kingdom of Italy operated entirely independently. (I suggest this book every time Theodoric comes up, he was awesome: Theoderic in Italy by J. Moorhead (Oxford University Press, Oxford, 1992).
This first Kingdom of Italy lasted to the sixth century. But starting from 535, the Eastern Emperor Justinian took advantage of infighting among the Romano-Gothic ruling class to re-integrate Italy into the Empire and sent an army under General Belisarius to depose the King in 553 and control the peninsula.
However, the Byzantine Empire had a chronic manpower problem and lost Italy in less than a generation. Although Justinian was definitely a great wartime politician who was particularly good at picking the right man for the right job, his conquests stretched Byzantium's resources too far. Even sympathetic chronicles criticize him for being unable to defend the empire once he expanded it.
Thus, the sparsely defended Italian lands were easy prey for one of the last great migrations: that of the Lombards. The question, "where did the Lombards come from" is covered in Jorg Jarnut's History of the Lombards (Enaudi, Torino, 2002) but that's a story for another post. The Lombards would contest Italy with the Byzantines from the 6th to the 8th centuries. The Lombards moved into the Northern Italian Plains with relative ease, while the Byzantines would hold onto the Pentapolis, the city of Rome and its surroundings, and the easily-reachable Calabria and Puglia regions of Southern Italy.
The Lombards eventually "Went Native", imitating the Roman Imperial customs, establishing Mediolanum (modern Milan) as their capital, while still administrating Italy in various feudal subdivisions; indeed, had things gone differently, perhaps Italy would have survived as a medieval state, and centralized as the rest of Europe did over the course of the renaissance.
But any peninsula that can sustain two ethnic groups fighting over it isn't going to have a normal history.
If one had to give a single and easy answer, it would be "the Pope". Starting with Lombards and ending with the Savoy family in 1871, there were countless attempts to unify Italy. All of them were countered by the Papacy, either stirring internal opposition in the enemy or calling for military intervention from European monarchs. On their side, they had the Faith, the Church organization which permeated Italy both socially and economically, and the offer of the Imperial crown. "Come to Italy, deliver us from our enemies and get crowned by the Pope" was an offer almost nobody turned down for a very long time. The history of Italy is, in some ways, the history of powers trying to rise over the others and getting overthrown by alliances sanctioned by the Pope. This all changed in the 1800s when the ideas of National identity rose and the political situation in Europe was highly unstable. The invasion of 1871 coincided with the Franco-Prussian War: the French would but could not intervene against the Kingdom of Italy, and the Austrians similarly refused to start a war to defend Rome, having been in to many unsuccessful conflicts in the previous years.