I really get a kick out of these threads. I'm going to sidestep the religion thing altogether because as one of those horrible, hateful Catholics, the OP isn't going to listen to me anyway.
There was a big huge thread about this last year, and it was pretty funny, but of course it got locked. Somebody (I can't remember whom) said something about every civ being in the game for a specific reason. I thought about this, and I think that I've figured it out.
Marketing!
Yes, that's right. Every civ is in the game because it fits into at least some of the following criteria:
1. It has been a significant player on the world stage, or it's omission would have people upset and they simply
can't leave it out. (America, China, Russia, the UK.)
2. It has significant historical value, either recently, or in the past.
3. We all learned about them in grade school, and for most of them we at least recognize the name of the civ, or perhaps it's leader.
4. The civ represents a region of the world which Firaxis felt was underrepresented in the game, and thus chose a country to flesh out that particular geographical area.
5. The inclusion of the civ or leader does not provoke a hot-button, knee-jerk, controversial response. The included civ offends the fewest amount of people. This is why Hitler and Pol Pot aren't in the game, for instance.
Civ was really made for those of us in the western world, and particularly North America, because that's where the money is. It's geared toward western culture and what most of us know of it from our history classes in school as children. Those of us who have gone on to higher learning may have had courses in history which filled out the details of the less than stellar accomplishments of Stalin and Mao for instance. Some of us are history buffs and know a bit more. Those are the people who ask the question "Why is civ x in this game and not civ y?" And they can give good reasons for each. Most people probably don't care, and the designers know it.
I honestly believe that Firaxis set out to offend no one, but still tried to give the game a "worldly" flavor, and a sense of historical importance. As an example: The Khmer. They were at one time the second largest empire in southeast Asia, but I'll bet that a lot of us had to look them up in Wikipedia or something. I'm willing to bet that they were included because the designers found a "hole" in the game. A region that had little or no representation. "Hmmm. We can't use Viet Nam, the Americans will freak out. Cambodia's a no-no as well. Hmmm." So they chose a civ to represent that actually did have some historical accomplishments, and were able to flesh out the region.
As for the Zulu, well, why not? Africa is poorly represented in the game. That might be because of what a couple of people mentioned above. Small, loose empires, or basically tribal societies with little or no cohesion, and that completely makes sense. Everybody has heard about the Masai, for example. I personally would like to see them included because their culture is interesting, their reputation for bravery is well known, and currently they are doing a number of things to help with the conservation of wildlife in the African habitat. They won't be included because their contribution to history is not enough, they are basically a tribal society, and they don't have a well recognized leader. Shaka has been in a few western movies, and many of us learned about the problems the British had in South Africa during the Colonial period. Most of us have heard of the Zulu wars. It's just brand recognition.
And remember, the game's designers have to submit the list of civs to the legal department before the game goes out to the store shelves. Given the fact that anyone will sue anybody for anything these days, I'm surprised that we have a game at all.