I don’t share the nostalgia for burbonic rule, but I have to point out that the fact that they lost does not mean that they were not invaded. Both things can be true.
In some cases it has to do with nostalgy, for whatever reason.
In most cases it has to do more with the fact that the South is primarily used as a way to steer/control the elections.
Garibaldi used local (criminal) lords, the "picciotti", to achieve what he needed. A bit like today they would use local criminals to subjugate the population. Those criminals stayed until Mussolini decided to deal with them. And again they got freed by the Allies to save the country from Fascism.
You could wipe out the mafia, make the land rich and use it, but there is not just enough interest to do so.
On the contrary, the only interest is to use that land to move to the next step, never to actually use whatever is there. This is what makes people nostalgic or sad.
So really, the nostalgics of Bourbon rule are just the Italian equivalent of American Confederates: they just never got over the fact they lost.