@unime well, no. I have no solid theory about why this island in particular is used. Rather I think the pantheon of accepted vs forbidden gods is tied to the conflict Abyss was made for. The system at its core feels like an excellent way to create highly effective workforce specialists. The theory is that this is a post apocalypse type setting, and that the actual source of the blessings is tied to the same people that made Abyss, and the actual history is obscured either by time and disaster or intentionally.
Maybe a particular subset of this blessings system was needed by the people who worked on the island way back when and that's why they were blacklisted/set aside.