Sicily is fertile, but for example much of Calabria is very much not - too rocky/mountainous. The issue is not that there's no fertile land, but the fertile land relative to the population is not a lot in terms of just sustaining the South subsisting on its own, and it certainly doesn't have enough extra fertile land to develop as much agricultural export from the region as the north has.

And it's gotten much much worse in recent years with climate change the heat and drought is getting very serious in the south.