No - it's because they live in a hot climate. Ok, poor joke ... but really, the inclusion of the Zulus reinforces, and is probably a result of, the traditional Eurocentric conception of Africa. I'm pretty sure Africans don't want to be represented by a people that completely fits the Western stereotype. And because the Zulus match everything we've been fed about Africa in imaginative imperialist fiction, they've also become boring. Bloodthirsty savages stopped being exciting in the 50s.
Personally, I had hoped for an interesting African civilization to be included in the xpac - like Abyssinia. The devs showed some creativity in including Mali instead of Zululand in vanilla civ4, but unfortunately they've gotten more conservative with their civ choices.