The two main reasons for the north/south economic divide of Italy is the availability of resources and economic investment. While the north of Italy had modest deposits of the iron ore and coal necessary for industrialization, the South was comparatively devoid of iron, coal, and copper, making industrialization a much more costly venture as any coal and iron would have to be imported, and bought at a markup through tariffs. The South however did have its economic blessings in the form of fertile farmland, which made industrialization an unprofitable venture for local investors in relation to the local economy.

When Italy became unified, one of the top priorities of the new Kingdom was to industrialize the nation so that it could economically compete with the more established powers like France and Great Britain. While France, Britain, and Germany had an immense natural wealth of coal and iron within their borders, Italy was comparatively lean in terms of industrial resources available in-house. Modest deposits of iron and coal were found in the Piedmont region of northern Italy, specifically in the area around Turin. The iron and coal from the Piedmont region would be developed to create an industrial triangle between Turin, Milan, and Genoa, where Italy's industrial companies such as Ansaldo, FIAT, and Aeronautica Macchi were founded.

While this is all well and good, Italy's resource wealth within its borders paled in comparison to Britain, France, and Germany, leading to Italy's industrialization becoming more localized to just the resource-rich north, while Italy's southern regions did not have any iron, coal, or oil deposits to speak of in order to industrialize. Italy ended up having to import iron and coal from abroad in order to industrialize further, leading to the process of industrialization becoming more expensive as Italy had to purchase the resources and the tariffs attached to them in order to keep their economy developing, slowing potential growth and limiting industrialization to areas where profit was more likely in the industrialized north rather than taking the risk of attempting to industrialize in the resource-scare south.

The Italian government attempted to alleviate its resource scarcity by investing in a colonial empire with all the other major European powers in hopes of securing more resources abroad, but during its 50 years of colonial expansion, Italy gained Eritrea, Somalia, and Libya as their major overseas colonies. Of these, only Eritrea had any substantial industrial resources in the form of copper, leading to Italy's Empire doing very little to alleviate its lack of industrial resources. Overall, this led Italy's industrial process to lag far behind its resource-rich peers, which I addressed in further detail in this answer over here.

When it comes to Italy's south though, its lack of industrial resources was made up in southern Italy having arguably the most fertile farmland on earth. Italy's famous Terra Rosa (red soil) concentrated in Italy's south makes the areas around Naples and Apulia among the best farmland to grow wine grapes, olives, and durum wheat. To economic investors looking at southern Italy, which would you rather invest in? Attempting to build heavy industry in a region with no local iron or coal to mine? Or invest in wine grapes grown in an area so fertile that a failed harvest is almost unheard of? This isn't to say that Southern Italy didn't attempt to industrialize at all though, as Southern Italy is home to Italy's first railroad, the Napoli-Portici line opened in 1839, as well as a successful textile industry centered in Naples. When it comes to economic investment however, southern Italy's blessings of fertile soil made agricultural investment the safer bet in the south rather than the comparatively risky investment in heavy industry.