Obviously laziness on the writers' parts might a reason, but whatWhat 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).