The six supreme clans were not the six supreme without a reason. Each of the clans had a trace of the sacred race lineage in their bloodline, hence the reason why they were stronger, more powerful, magically inclined, and darker than the other races.
The devil, demon, and vampire race for instant had traces with the netherwinian race, while the eagle race had traces to the phoenixia race. The werewolves had traces with the lightixia and phoenixia races, while the serpents had traces to the draconian race.
But these races' sacred race linage bloodline was so thin that if the other races worked together, it would not be difficult for them to bring one supreme clan down and replace it. However, the six clans auras acted as strong deterrence for the lesser races for them to attempt such a feat.