Not all counties apply the same warscore. Not sure how it's decided, but my guess would be its based on contributions to main belligerent. I imagine many will be more like 4 percent in that region, but I can't be sure. The wiki mentions it's based on the defender's holdings, but I don't know if that means demesne or total, or some mix, or what. Regardless, I think this is a case of just needing to go fight the Crusaders. I'll be honest, they can be remarkably incompetent, if you can get a large enough army to smash into them on the far side of their carpet sieging. It'll take a while for them to actually fully get into the battle and that can be the end of them.
Honestly, it is kinda weird that there isn't a greater emphasis on the war target holdings in a Crusade, but it is what it is. You are the main defender, which usually means you're the biggest one (not always), so if they just siege stuff down unopposed, your realm and military would just slowly collapse as chaos spread. You need to beat them up to protect your realm, or else internal pressure and dwindling military resources will force you towards defeat.
Although, do keep in mind, you will get +1% War Score per month once the ticking war score starts from them not holding any of the war target. Considering it takes several months for them to siege down a county, it may end up that they have a war score deficit if they siege just one county. Though they will likely siege multiple, that does end up taking the edge off of their progress and means they have to occupy a lot more counties that just 14 or 15.
PS: Have you considered just letting Asatru lose and give up Bulgaria? If it has starting de jure, it'd only be a 1 or 2 county lose overall, which is honestly a pretty cruddy Crusade all around.