When it comes to the main reasons in short; their late unification in 1861 meant that other, more established powers already held geographically important strongpoints and dominated world trade before a unified Italian state could even attempt to make a stake in such matters. The other was Italy's overwhelming agrarian economy upon unification in comparison Britain, France, and Prussia/Germany. That said, Italy was considered among the great powers of Europe upon unification, with victory over the Austrian Empire during the Unification Wars being the main claim to take Italy seriously on the international stage upon its foundation.

It's true that while the Italian merchant republics were wealthy during the medieval and renaissance eras, dominating Mediterranean trade and were the main providers of exotic goods from Asia such as spices, porcelain, and silk. The more lucrative trade routes had yet to be opened with the discovery of the New World.

With Spain and Portugal, and later England and France setting up colonies in the Americas while the Italian states never were able to break out of the Mediterranean, the republics remained wealthy, but the new colonial powers became exuberantly rich in comparison. New trade goods from the New World such as tobacco, cocoa, and sugar began competing with the goods coming in from Asia. And as Colonial Powers began setting up colonies in India/Indonesia/the Philippines, the main consumers of these goods could skip trading with the Italian states entirely, which was a hard hit to their economies.

Next is Industry; simply put, England, France, and Prussia/Germany started investing more in mass production of manufactured goods while a divided Italy had little means to compete in terms of production and construction of industrial centers. Rail lines were late to be constructed in Italy due to the lack of connection between the separate states and the money for these massive building projects simply was not available to these smaller states by the early 1800's.

The lack of natural resources within Italy's borders also presented a problem with industrialization; with very little in the way of coal, and later a complete absence of oil, further perpetuated Italy's agrarian-focused economy into the 20th Century. When you look at Italian military equipment from the 30's and 40's, especially their tanks, their armor is entirely riveted and bolted as opposed to welded like American and German armor. This wasn't in ignorance to the benefits of casting and welding, but simply because Italian industries such as Fiat and Ansaldo didn't have the welding and metal cutting tools necessary to mass-produce welded metal while Henschel and Chrysler did.

Despite Italy's shortcomings in comparison to its neighbors upon unification, Italy was still considered a major player in Europe and a serious player in politics. With Italy gaining a colonial empire in the scramble for Africa in Somalia, Eritrea, and later Libya, scoring military victories against Austria in the 1850's and the Ottoman Empire in 1911, and ultimately, victory in World War I did much to boost Italy's standing on the world stage. But the increasing pace of motorization and industrialization caught up to the county by WWII, with the country having no means to compete against industrial titans like Britain, the US, and the Soviet Union.