It's the law of preferential attraction, or the rich get richer. In the case of Battlemaster, realms that get big become more powerful, allowing them to get even bigger, and so on. In the beginning, when every realm is small, multiple small realms could exist on that peninsula. But sooner or later one realm among them would begin to snowball and gobble the others up. Sooner or later there wouldn't be any realms to challenge it, it would just be one big realm isolated from the rest of the continent.