I think it might have things to do with many of us falling in one age bracket. Anime experienced a boom of mainstream popularity when most of us were teenagers and our interests were shaped. Many other online communities are either "densely populated" by current teenagers that are chasing current trends (mainstream anime popularity is kind of a thing of the past by now), some are conversely frequented by older people that were already "too old" when the anime boom happened.
Plus anime, as well as videogames, is a popular source of escapism and provides a source of solitary entertainment, and with many of us seeking an escape and not being very social, it's a popular option.