[City of Avalon, Central Tower Deepest Floor]
Even though the triad's popularity is not that defined in comparison to the archangels, which caused many people to think that archangels are the top of hierarchy in heaven, it is still a given fact that the principalities is atop of the hierarchy much more than the archangels and posed as leaders in the third trIad group. All in all no matter how much powerful and defined the archangels are, they are still part of the bottom ladder and far from being close to omnipotence.
It is like the comparison of a soldier in comparison to a politician. Even though soldiers are more adept in combat than the politicians itself, the former would still need to listen to the order given by the latter due to the difference of importance between them. A form of hierarchy in power or system established to form an order of things.