I think the estate systems are to blame. And that's kind of weird for me to say because I love the estate systems, the old one even more than the new one.
The thing is, in the old estate system you sometimes couldn't go to war since you had no nobles to support any conquests, and land is and has always been THE most important motivator for war. (though in my opinion that's just a common excuse, take Thalmarkin before the 4th invasion: we didn't have enough nobles so we just let 3 of our badlands go rogue and went to war to take a strategically important stronghold, and of course... kick some ass.)
Anyway, with the new estate system that is no longer an issue. The issue now is: why try expanding when you would probably profit more from having your nobles concentrated in a city than by taking those rurals? Why risk it all for little gain?
Again, that is a stupid excuse in my opinion but I do believe it is, again, a common one.
The fact is that ever since "conquest" lost it's unquestionable correlation with "profit" or "fun" (because yes, it can be a drag to have to control too much land, or to build up newly conquered land again) wars became less appealing. Of course there's still those people who don't care and just want to have war for the fun of it, but I think there are also the more calculated players who don't immediately see an objective reason to go to war anymore. And thus stay at home.
I think the failure of TMP was that it tried to punish those who stayed at home (and those we did go to war but didn't manage to scrape up enough glory to last for one refit period) in stead of trying to encourage them to go to war. People would think: "Oh, TMP will never happen to us, we're still fighting monsters here and there!" but then it did and they'd go: "Darn it! Now we can't go to war anymore! Our units are completely useless!"
I don't immediately know how to solve this without getting rid of other great aspects of the game but I think the lesser value of pure land is one of the most important reasons for avoiding war.
I am thinking though that under the new estate system less efficiency means that less gold is getting collected by you or your knights but that gold needs to go somewhere... Maybe the peasants get richer and there is sort of a build-up of wealth inside the region that could be accessed by special, hurtful, instant and direct taxes and looting? Just an idea though. Land would at least not lose it's value anymore, it's gold would just not immediately fall into your hands (unless you have 100% efficiency) and it would increase income from looting, another of those fun acts of war.
