This scam is very old. My father described a version of it to me when I was 12.
In the early 1960s he knew a widow in Germany and she got a letter explaining about her husband's account in Nigeria. Her husband was a former oil exec and she obviously found it convincing.
The key to their scheme then was to get her to come to Nigeria where they would arrange a little drama to get her past customs so she wouldn't have to declare coming back out.
Once she was in illegally they just extorted her by threatening her with jail.
So in this case there are clear reasons
1. They'd ultimately like to get the mark on site as it were
2. Everybody thinks Nigeria is corrupt so they believe dirty money will exist there in large quantities
3. Colonial contempt for Africans traditionally makes older people of a certain attitude susceptible to the idea that they are dealing with less savvy people than themselves.
It's much older than that - it dates from the 16th century [1]. I have an original example from the late 19th C. that was sent to one of my ancestors, so I did some investigating when it came to light.
Fantastic! There's a book by Charles Stross called "Neptune's Brood" (https://en.wikipedia.org/wiki/Neptune%27s_Brood) where the central character is a historian specialising in these kinds of frauds. This includes futuristic versions where the 'Spanish Prisoner' is offering 'slow money' to in return for 'fast money' to help finance a new FTL drive:
> They're not poor: Here see these slow dollars signed by a bank a very long way away? Won't you hold them as collateral and front me a sum of fast money to help my friends make their repairs? We'll accept a ruinous conversion rate, just in return for the money we need to get our space drive working again; when the bank-countersigned certificates for these slow dollars reach you, you'll be rich!
I scanned it and transcribed it and shared it online a few years ago, but not sure if that is still around, and I'd have to dig through boxes in the loft to find the original, however, the letter this article [1] describes sounds almost identical in content to the one I have.
It reminded me that I also have a "letter from the prison chaplain", and a short follow-up letter from "Antonia Garcia" in shaky writing that trails off mid-sentence, and another letter from the "chaplain" saying he had passed, enclosing a supposed prison death certificate.
Apparently another scammer, using the same pen-name told his 1905 victim he'd been scammed, and that he'd be laughed at if he went to the press or authoritiees:
Interesting Q - was Spain chosen as having similar role then to Nigeria now? Lots of gold. Maybe viewed as lawless / corrupt? (Inquisition). Possibly contempt for Catholics?
Interestingly, there is even a movie called The Spanish Prisoner (http://www.imdb.com/title/tt0120176/) from 1997 based on this type of scam. It is good and I recommend it.
The movie plot has very little to do with the spanish prisoner scam. A good movie anyway, probably Mamet's best after Glengarry Glenn Ross and House of Games.
Its so dumb that people in todays day and age under estimate the intelligence of the Nigerian people who as a group in American are easily on of the most educated groups around with >25% of them have a graduate or professional degree.
(http://www.nytimes.com/2014/01/26/opinion/sunday/what-drives...)
In the early 1960s he knew a widow in Germany and she got a letter explaining about her husband's account in Nigeria. Her husband was a former oil exec and she obviously found it convincing.
The key to their scheme then was to get her to come to Nigeria where they would arrange a little drama to get her past customs so she wouldn't have to declare coming back out.
Once she was in illegally they just extorted her by threatening her with jail.
So in this case there are clear reasons
1. They'd ultimately like to get the mark on site as it were
2. Everybody thinks Nigeria is corrupt so they believe dirty money will exist there in large quantities
3. Colonial contempt for Africans traditionally makes older people of a certain attitude susceptible to the idea that they are dealing with less savvy people than themselves.