What would you build with $20B near Milan? Also, it's not like Northern Italy is forgotten when it comes to government investment. The Brenner Base Tunnel is being built there which will probably cost on the order of $15B or more.

Per-capita GDP is very low for Sicily relative to other regions - but it still has an overall GDP of around $100B, which is similar to Costa Rica or Croatia. Giving it a car/rail connection to the mainland would be a huge boon for the region and Italy in general.