Why do Many of the Phishing Schemes seem to focus on Africa?

Why do so many Phishing E-mails seem to be from Nigeria. Is there something about those particular nations that make them a particular target? I have seen Several Iraqi items as well however the reason they use that is pretty obvious. I just don't see Nigeria as wealthy enough to be throwing cash around. So why this gimmic? if your going to lie like hell to get money I would think you want people to be able to believe it for it to work.