Well, there is no mystery, the reason is the same as for all the exodus from (mainly) South of Italy between the end of 19th and beginning of 20th Century, even if it lasted a bit until the 70s. Search of places where they could find work because of lack of it and poverty in the original places.
USA, Argentina, Brasil, Venezuela, Uruguay , (Peru) and eventually Canada and Australia offered them a possibility for a new life beacouse these Countries in different fields were "offering" labor. Many poor Southern Italians in the 50s-early 70s also went to Northern Italy, Germany and Switzerland.