From what I gather, Battlestar Galactica was shot on film, Kodak Vision stock to be precise.
Grain can be much more noticeable in dark areas with certain stocks and film speeds. Sometimes it can be a compromise between the stock looking fantastic in the majority of lighting conditions used in the film/show, versus that of dark ones.
It could also be a mistake made by the DOP and it's not unheard of either for the negative producer to make claims about the film stock only for it not to be as good as hoped, or a certain batch wasn't so good.
As somebody else pointed out, it's similar to how with films shot digitally will tend to have a lot of noise in the dark areas when using high ISO which introduces a lot of it. In general though, grain looks far more pleasing than digital noise does.