Leyrann
Deity
It is worth noting, that everybody has always been fine with including comparably very tiny and humble native nations of North America (total population of Iroquis was no higher than fifty thousand people), whereas the requirements for Africa seem to be some super special turbo important states.
I'd say it's probably mostly because Americans (obviously) make up a large percentage of people online, and they're more likely to be familiar with local civilizations of the past than those of another continent.
Another thing that might also contribute is that Africa at least has some major states in the past, whereas I'm not sure whether north America (as in, north of Mexico) ever did. Admittedly, I barely know more about pre-colonial north America than aforementioned people do about pre-colonial Africa...