It could be argued that Diraan’s primary defense is in not being worth the effort to conquer. While it is hardly a barren land, it is not uniquely gifted above its neighbors. Added to that is the terrain. Diraan is located in the High Synd-Galir, a high plateau of rugged terrain, difficult to assault. For Ornestra to conquer, they would require so many troops that they would leave themselves vulnerable to other neighbors, especially with Virstachreon pledged as an ally.
The Diraani are not a united people, preferring local rule among the various tribes. When threatened, however, the tribes will gather and choose a Grand Diraani, a warlord who can unify the people to face that threat. Any given tribe can be formidable. All of them, operating as one, is truly terrifying.
Though a wild people in a wild land, their lack of sophistication is superficial and deliberate, more of a statement of their disdain for the dissolute and overly-complicated lives of their “more civilized” neighbors.