Humans are a varied bunch. We never cease to be amazed by the difference and number of cultures on this planet, with each culture having its own facets that are often baffling to other culture. In fiction, however, this never applies to other races.
Basically, a given species has a single, unified culture. Everyone speaks the same language, the species art is uniform (ie: you have Klingon Opera, but no difference between Homeworld Klingon Opera and Colonial Klingon Opera), there's only one religion, and of course, one government.
This is also present in fantasy setting, (and thus different from One World Order
) because every species in the world has just one culture. Dark Elves have their one culture, the Orcs have their own, etc... with generally only humans showing any variety.
Planet of Hats
is a subtrope of this, when the alien culture is focused around a single, specific theme. Sometimes, this is justified
with humanity having a "hat" of multiculturalism or diversity - it's not other races who are oversimplified, it's humans who are complex.
Any subgroup presented as having a different culture than the norm of their races will always be treated as a strange minority, perhaps even shrouded in rumors or an example of Defector from Decadence
or My Species Doth Protest Too Much
See also Planet of Hats