Depending on the setting, that could make a lot of sense. Imagine a planet settled entirely by the descendants of a single expedition. That planet wouldn't be a complete cultural monolith; not everyone would be identical. But an entire planet with the cultural diversity of a small place like Iceland really isn't unreasonable. If it's a species' home world, that makes less sense.

Or, a really dark bit of head canon? Every time you find an alien species that lives on its home world and has a single culture? Inevitably this means a cultural evolutionary bottleneck existed in the planet's past. If it's not a colony planet, then something else must have caused that bottleneck.

My head canon? Any planet like that is one where an alien Hitler won. When you encounter a planet like that, it means that some time in the last thousand years or so of that planet, a Hitler-like figure came to power and achieved global hegemony. They decided that there was one and only one right way to live. Everyone was either forcibly converted to that lifestyle or done away with.