Mostly because of distant travel, but we DID actually hold them off for a while.
I don't think setting regions to rogue will help, I think what could do is to decrease their control, lordless regions can only have province control and make it harder for regions with a lord to drop to province.
This would help a bit, and would make it easier for other realms to take those regions.
The main problem is: there is not enough players for more realms, setting regions as rogue will not help and will only make people fight the monsters/undead that pops in from them without actually going to war.
One side being able to hold off the other doesn't make it an even fight. IVF and Nothoi might be able to hold off Rio from the core regions, at least for a while, but they would rarely get a chance to press the attack, and when they do, it will rarely be effective. Same with Luria. Luria vs Morek, alone, and Morek will always be on the defensive. It was pretty tied when I left, with everyone united against Luria... and yet they still managed raids into Morek regardless.
Distance aids defense, but doesn't make fights more balanced, because in the end, the stronger party will have all of the leverage he wants in the negotiations to end the war (or not).