Why do Nigerian scammers say they are from Nigeria? I have no idea but these Microsoft researchers say they do. The link is to a 14 page .pdf.
Since gullibility is unobservable, the best strategy is to get those who possess this quality to self-identify. An email with tales of fabulous amounts of money and West African corruption will strike all but the most gullible as bizarre. It will be recognized and ignored by anyone who has been using the Internet long enough to have seen it several times. It will be figured out by anyone savvy enough to use a search engine and follow up on the auto-complete suggestions [of search engines]. It won't be pursued by anyone who consults sensible family or fiends, or who reads any of the advice banks and money transfer agencies make available. Those who remain are the scammers ideal targets. They represent a tiny subset of the overall population.