Because they have this thing called Perdan on the other side of the continent wanting to wipe them out.
Also, have you even seen their list of dukes? All insanely loyal to Sirion. Unless Perdan fragments, Sirion won't fragment. The only reason Krimml defected was because well the duchess there was the infamous Relak family. I wouldn't be surprised if the duchess changes her side again when things start favouring Sirion.
