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.
To figure out which angle is more correct, you need to consider facts about the specific regions.
Backwards regions are subsidised by the cities. Sicily can be made into a tourist resort for pensioners.
Or perhaps building a bridge will enable them to bring water in easily from the mainland?
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.