I mean, after the fall of the Western Roman Empire, the Italian Peninsula was split into various kingdoms, duchies and parts of other empires, and they didn't unify until the 19th century.
From what i can tell, right after the collapse of the Western Roman Empire, there was a short lived Kingdom of Italy, but after that they were fragmented until the 19th century.
How come it took them so long, since from what i can tell and know, the people of the italian peninsula, were more closely tied together, ethnically, culturally and linguistically, then the people in other regions, such as Spain, France, Germany and the British Isles. Of course i could be wrong.
Well, if it it had been clear how to unify Italy before 1864, someone would have done it!
But no, in all seriousness, it's a combination of factors. A bit like the fall of the Roman Empire, the reason can be whatever you want it to be. I narrated the history of Italian unity and disunity in this absurdly long answer from a long time ago, which I will summarize and narrow down to driving factors below.
Due to word count constraints, you can find the full body of the answer in the reply to this comment. In summary, Italy existed in a space with enormous social and political baggage: it was the place in Europe where old Roman institutions were strongest. Generally, in all of post-Roman Europe there is a transition away from the "Politics of Taxation" that had existed in the old Empire, and a move towards the "Politics of Land." In some places, this transition was very rapid and dramatic; in others, it was more gradual. One of the places where the shift was weakest was Italy.
These same legal and political constructs meant that Italian politicians were quick to adopt the framework of the Holy Roman Empire on top of the existing body of Roman Law in order to guarantee the existence of highly autonomous urban communities fostering prosperity and order, favoring subterfuge and diplomacy over conflict (when possible); ensuring laws were well-developed and fair; and society was stratified, but also complex and vibrant (thus according to most widely accepted economic estimates, Italians had the highest average income in Europe up until the 16th century). The Italians didn't necessarily need to be adopted by a foreign Empire to do this, but that's what ended up happening.
Given these conditions, the prospect of unifying the peninsula was only interesting to the Italian ruling class in that anyone who tried to do it needed to be stopped.