Why invest in the region that’s already developed when you can invest in one that still has room to grow? That’s the meaning of investment. It seems likely that Sicily is poorer, in part, because of the lack of a bridge to the continent.

The opposite angle is that investing is a well functioning region gives better results that investing in a corrupt and dysfunctional one.

To figure out which angle is more correct, you need to consider facts about the specific regions.

But Milan is a sure bet? You know that you'll make the money back.

Backwards regions are subsidised by the cities. Sicily can be made into a tourist resort for pensioners.

Only if Sicily can somehow make water magically appear (I won't bother linking any articles, as there are literally hundreds in the last 2-3 years about the utter lack of water and suffering of Sicilians).

Or perhaps building a bridge will enable them to bring water in easily from the mainland?

The water issues are less due to actual desertification (it's a contributing factor too, of course) and more caused by the ancient and perpetually leaking pipes infrastructure. Way more compromised on the weastern side than on the opposite part.

Speaking with the experience of someone who has spent a good half of his life on that island: Sicily's problems are mainly due to the following factors, in this order: culture and mismanagement. Everything else (yes, including the organized crime) comes in a very, very distant third place.