Why is is that fictionkin only seem to appear in large fandoms with a cult-like following, such as Lord of the Rings, Avatar (James Cameron), Supernatural, Homestuck, or the wide array and otaku culture of anime/manga?

The key word here is ‘seem’. There are plenty of kin from other sources, you’re just not interacting with them. However, one reason that sources with large followings have more fictionkin is this:

The more people who see a source, the more there are to connect with it. There may be plenty more people with souls from ‘obscure’ sources out there- but they never see them. And so they never recognize exactly what it is they’re feeling to identify as it. To identify as fictionkin you have to have seen the material in the first place, otherwise all the memories in the world won’t make you know what’s going on.

For example, someone may have feelings of having been stronger than 10 men, and remember snatches of a childhood being raised on a farm, and remember being a journalist- but if they’ve never heard of Superman, how are they going to say ‘Oh, I’m fictionkin, I’m Superman.“