This phenomenon isn’t confined to Italy. You see it across Europe, the US, and (reversed) in Australia. Charitably, you could chalk it up to sunnier regions being better for agriculture and not cities, but I think that heat simply causes the human brain to thermally throttle, which makes humans in hotter climates slower (and dumber). And less enterprising, and wealthy. And certainly humans need to be more resourceful to survive in areas with less food available. Is it a coincidence that the last uncontacted tribes are all in the equatorial belt? Coelacanths of the modern world!

More speculatively, Europeans in particular could have been subject to extreme selection pressure during ice ages. Glaciers made it all the way to central Italy, which would have definitely shaped the evolution of humans, both biologically and socially, in those areas.

Finally, humans are less fertile at higher ambient air temperatures, and the risk of death greater than in milder climates. A population boom spurs all kinds of cultural and evolutionary and epigenetic changes.