I have some NZ perspectives to put through here. In the NZ gold rushes, most of the Chinese migrants were Cantonese, from Guangdong and Fujian, even though NZ was British at the time. So I would put it down to a tradition of migration, although most Chinese in New Zealand now are Mandarin-speakers. At least in Auckland, where the vast majority of Asian New Zealanders are.
Also, Croatian migrants that came to New Zealand came from a relatively small area of Dalmatia. Dalmatians too had a tradition of migration, evident in the large American, Chilean, Australian, South African and New Zealand Croat diaspora, and almost all of these migrants were Dalmatians, rather than Pannonian or Slavonian Croats. Each section of coast (pretty much every cluster of villages) migrated to a different area. For instance, the Head Boy of my high school (which actually has a very small number of Croats) was descended from migrants from the same village as my family, and we came across a book by a great-uncle, which mentioned his best friend, who had the same surname as the Head Boy!
Although that seems like I digressed a bit, I have a point. It's chain migration. If a relative or neighbour has gone to another country and done alright for themselves, you'll think "why not?" and go yourself. That's why ethnic diasporas tend to be from a close geographic area. I don't know too much about it, but I'd assume that most Irish-Americans could trace their lineages to the same area, and AFAIK most Italian-Americans in the US Northeast are Sicilian rather than Lombard.