This is a very good question.  History, I suppose, on the one hand, at least in the United States.  When the American railroads were built, they were built with a great deal of (inexpensive, read underpaid, sometimes not paid) Chinese labor that had been imported from Guangdong and Fujian.  Most of these laborers remained in the United States afterwards and went on to found the Chinatowns that now exist in San Francisco, Los Angeles, New York, Chicago.  They in turn brought over their relatives and those relatives brought their relatives and friends.  In the United States, until very recently, this kind of immigration was favored by the family re-unification provisions of the Immigration Act.

They came from Fujian and Guangdong mainly because when Britain hand the concessions in Xiamen and in Guangzhou and in Shantou and in Hong Kong it built local railroads with the help of local laborers.  The Americans noted this when they went on their quest for (cheap) labor.

Even today, I would venture, and only venture, to guess that probably 80% of the Chinese restuarants in the country are Cantonese restaurants.

Case closed, 981 has  it all, one supplyment Fujian and Guangzhou area has stronger big family tradtional.