It's important to note that there were two primary forces behind Italy's unification: the first was nationalism, and the second was military strength. Italy had long been divided between many polities of relatively equal strength, in areas not dominated by strong foreign powers that is. So unifying the territory of modern Italy required a state of significant martial strength to dominate both the foreign controlled lands and the other states within Italy itself. The only claimant to this during the early modern period, when those other states you mention were uniting, was the Kingdom of Naples. However, it's repeated attempts at controlling the whole peninsula were blocked by foreign powers and eventually it's throne fell to one.

Fast forward to the mid 19th century and the Kingdom of Sardinia (actually mostly based in Piedmont). Most of Italy's old older had been swept away by the Napoleonic wars, and what was left were fewer states, but with rulers who had weak claims to legitimacy. Except Sardinia, who came out stronger than ever (in the end) from the wars in terms of legitmacy, military power, and most importantly, territory. In addition, their ruler had the legitimacy (the House of Savoy being one of the oldest ruling families in the world, and native to the Italian region).

At this time, there was also an upswing in nationalism throughout Europe, particularly of a republican type. Nationalists rose all throughout Italy, in both domestic and foreign controlled areas. The years of 1848-1849 saw actual revolutions and armed conflict between states and nationalist revolutionaries. Indeed, in both Venice and Rome, revolutionaries actually took control for a time, necessitating armed response. These revolutionaries strongly agitated for cultural unions in Italy, Hungary, Germany, and elsewhere, giving grassroots support of the people for such actions, should someone reach out and take it.

So, the stage was set. Except for those pesky foreign powers. You see, Austria controlled most of northern Italy thanks to gains they made at the end of the Napoleonic Wars. So, Sardinia first took Lombardy (the area around the city of Milan, a very rich area indeed) with the assistance of France (trading away a large area of Savoy in exchange), although they didn't succeed in taking the city of Venice as well. This left only Sardinia, the Papal States, Naples (Kingdom of Two-Sicilies), and Austria (Venetia) in control of Italian territory. Both the Papal States and Naples were extremely weak, and fell to little to no resistance, as the one was anti-thesis to the democratic and cultural fervor now embracing the peninsulla, while the later was controlled by a foreign installed king with no love between him and his subjects (and actually fell to a citizen rather than organized invasion!). So the Papal States (except for Rome) and Naples fell really quick, and bam, Italy is unified. All it took was a strong state establishing dominance, the support of newly popular Italian nationalism, and the weakness of foreign powers. The rest of the unification process (Venice and Rome) took place when Austria and France (which occupied it to protect the Pope during the revolutions of 1848) to be weak. Both thanks to Prussia. And speaking of Prussia, you mention European countries that unified sooner? Well, Germany unified later than Italy, and its unification is largely explained by the same factors. For all intents and purposes, both it and Italy unifying are probably seen more accurately less as unification and more imposition of one polity onto the rest, under the auspices of nationalistic fervor being exploited to cover for it. In Germany's case, it was Prussia doing the imposing.

Italy wasn't the only case. Germany also came together in 1871, the same year that Rome finally became the capital of a Unified Italy. There were a couple of different reasons for this. One of the big ones was foreign intervention. The Hapsburgs, Spain, France, and the Papacy often propped up one town government to pursue their own agendas where the other players propped up a neighboring town to frustrate those aims. That makes it hard to come together in common Italian-ness, when you've been fighting your neighbors for a long time (and have been getting PAID to do so). The Council of Vienna in 1815 settled a lot of the border disputes and is the start of the modern political model in Europe.

There really wasn't an Italian identity until the same time period. Remember even during the Roman period there were Romans and other tribal peoples. Then you started mixing Germanic Goths and Lombards. On Sicily you had some pretty impressive mixing with the Island being ruled by just about everyone from Normans to the Byzantines to North African Muslims. You can see the traces of this even today as the cuisine and culture of Northern and Southern Italy varies significantly. Only after the French Revolution, or more accurately the Italian Campaigns of the Napoleonic Wars, disrupted the more or less stable legacy city states and feudal realms that people started really thinking of Italy as a thing that really existed, prior to this it was noted by Austrian Diplomat Klemens von Metternich stated that the word Italy was nothing more than "a geographic expression". Over the next few decades the language, culture, and political structures in Italy began to standardize in ways that hadn't been seen before, books like Alessandro Manzoni's The Betrothed were written with an eye to bringing dialects closer together, along with Niccolò Tommaseo's Dictionary of the Italian Language, and the works of artists like Ugo Foscolo and Vincenzo Monti went a long way to bridging the gaps between the different Italian states.

Really one of the big problems with unifying Italy was the Papal States. Going heads up against the Pope was always risky, and as long as the Pope was also the functional King of a swathe of Italy from sea to sea there was no way to get the Southern Italian powers of Naples and Sicily together with the Northern Italian states. In fact, that was the final stage of Italian Unification as it actually happened. Even then the Pope locked himself into the Vatican and refused to acknowledge that Italy was a thing until Mussolini gave him a face saving out.

It's important to realize that Nation States as we think of them are relatively recent development, and it wasn't until the First World War that the personal demises of the Hapsburgs (the Austro-Hungarian Empire) split into something resembling a modern state. Italy wasn't late, it was riding the same wave of modern state formation as everyone else.