[Cohen]

Because their best mage, the Harab Elder, costs far more than an Arch-Seraph, and is capitol only.

Meanwhile Caelum could recruit in every of his castle an Arch Seraph ...

I don't see Caelum inflexible at all.
It's like most nations, with his best mage with 1 random pick.

And I don't think their troops are weak.
Prec 12 Archers, with wind guide base spell, and a Seraph, are good enought to conquer no knightly provinces.
Even you can avoid knight provinces flying around and over.
You can avoid to get bottled and lost turns coming back into your provinces.

Caelum troops are expensive in resources too because:
A: Their armor raises protection in cold provinces.
B: Almost all their weapons are considered magic.(get rid of Etherealness, break mistform).