Among the New Gods in any given realm, there is always one who is at a state of greatest harmony with the demesne. They shape the dimension with their will, even as they are shaped by its energies. This individual is the Primarch. In many respects, a Primarch is a ideologue for the very concept of their domain.
Primarchs are not driven by the same motivations that afflict mortals. They are created because they are necessary, an 'anchor' for the thematic aspects of their realm. And the realm exists to give them a place to stand. Places like Asgard exist partially in the reality of space, and therefore are subject to many of the laws of physics and the 'rules' of magic that govern it. Somewhere like Limbo, on the other hand, is a chaotic mess and responds rapidly to the will and intent of the person who directs the harmony of that place.
It is not impossible to overthrow a Primarch. There are rules for every demesne. In some realms it's possible to slay a Primarch and simply take their place. Others would require bending to the harmony of the realm and garnering the support of the other residents. The only consistent claim to the power of a plane is having the force of will necessary to do so..