Why do alien cultures tend to have one language, one religion etc

Question

What are some common logical reasons given within any sci-fi universe for the lack of cultural diversity within a single alien race?

Examples of what I mean:

• Star Trek: Vulcans come from Vulcan and speak Vulcan. Their history gives a good reason for their dominant religion/philosophy, but AFAIK they spoke Vulcan before the Surak days.

• Babylon 5: Minbari come from Minbar. They only have 3 languages (better than 1), but all have the same religion.

• General "{species name} culture". Klingons are spoken about as if there is only one Klingon culture. Conversely humans are very varied in our cultural beliefs. The value of honor and pride to the Japanese is significantly different than it is to most Westerners. Hindu religion is very different than Christianity. There is no "Human culture" (though in shows, this term tends to refer to Western culture).

Answer 1

Because the other question is being used as justification, I'll call upon the Power of TVTropes as well.

One World Order:

According to a good deal of Speculative Fiction set in The Future™, it is the natural order of things that all governments will merge together to create a central authority to govern the entire species. It's not necessarily the human species, however.

Sapient aliens also almost always have a single government to whom every law-abiding sophont in their race answers. Any conflict between members of the same species will be called a civil war. Especially true if The Verse of the show contains boatloads of sapient species.

That said, I'll also copy what I put in the comments and expand on it a little. It's not quite as simple as you make it out to be...

• The Narn from Babylon 5 have several religions, and are open to atheism. A new religion on their planet was even created accidentally during the course of the show, with a new religious figure who hates the role he's ended up in.
• The Vulcans and Romulans both came from the same planet, and had vastly differing religious/cultural views. But even after the split, before the Kir'Shara was found, there were multiple religious sects on Vulcan with different interpretations of Surak's teachings. The most prominent was the Syrranites, perhaps that name sounds familiar...
• The Klingons had their strange offshoots as well, such as the group that headed off to the Delta quadrant in search of their savior. They encountered Voyager in 7x14, Prophecy.

Now, there's one thing I'm sure you'll notice from these, that would seem to support your claim: They (almost) all still hang on a core belief system from that culture. The Vulcans believed in Surak, but split off, the Klingons in Kahless, and the Narn started with G'Lan, adding more prophets through the years.

This is very familiar to most of us in western culture: This is what happened to Judaism, with one branch splitting off to become Christianty, and then it happening again with Christianity to create Mormonism.

Write what you know, eh?

Answer 2

In-universe this may make sense for space-faring species. Once a species ventures out into space and comes in contact with alien civilizations it should become easier to think of everyone on one's planet as "us", and the aliens as "them". This should contribute to the emergence of a single planet-wide or species-wide culture.

Conversely, humans seem to have a psychological need to divide the world into "us" and "them". Until and unless we meet some aliens, the "them" have to be other humans, which contributes to our cultural diversity.

Answer 3

The likely in-universe answer would be that the culture/class/caste/religion/faction that the viewer is exposed to through the course of a story is the dominant one within that race or group. This would be much the same as how, from an alien point of view, western/christian culture is usually the one associated with humanity.

One notable exception I can think of off hand to this is the Klingon culture as portrayed in Star Trek Enterprise. Through the course of the series it is revealed that there are multiple dialects of the Klingon language (think how English, Chinese, Spanish, Russian, etc could be seen as different dialects of the human language from an outsider perspective), as well as at least two separate castes (healer and warrior).

If you are looking for out of universe answers, the reasons provided in the comments should already suffice (lack of space and/or time, lazy writing, etc).

Answer 4

There is no "Human culture"

This seems a shallow view to me. To offer the same opening as David Foster Wallace,

There are these two young fish swimming along and they happen to meet an older fish swimming the other way, who nods at them and says "Morning, boys. How's the water?" And the two young fish swim on for a bit, and then eventually one of them looks over at the other and goes "What the hell is water?"

Surely what's strange is that every other "alien" culture is so very human-like.