Because there isn't any other major way to do major economic damage to another country, and there isn't any other real way to force anyone to "submit" in the game. Most activity happens in cities, not in cabal headquarters. This kind of raid would involve not only the tribunal, but any citizens or like-aligned visitors that are around that want to help defend. I actually don't foresee it being boring, I actually think it would make the CRS system a boon to the game as opposed to how it seems to be right now: an afterthought that's been almost all but forgotten.
Also, because CRS right now is pointless and this will give a point to the CRS code. It moves the focus back to cities.
Conflict due to RP reasons is fine. I never said party vs party would become obsolete. Raiding the city to deal visible damage is actually a dangerous thing with this suggestion, because you have to deal with whatever defenders are around, any spawned guards AND any other people who decide to jump in to help.
As a side note, I think to gain entry to the "core" of the areas (city hall statue, bank vault, headquarter barracks), you would have to kill two fair-leveled guards at the entrance and the person barring entrance (sergeant at headquarters, head teller at bank, judge/magistrate at city hall) would have the key. Once they are dead and the key is lifted, you enter "raid" mode. Tribunal members aren't alerted until the Judge is engaged in combat. To facilitate things, the Judge and the guards do not aid anyone in combat but their own group (they are not marked as 'cityguards' per se). This is so that an outlaw running around doesn't accidentally start a raid.
As a side note, move the prison into the city halls in a separate 'corridor' and have a JAILOR NPC hold the key instead of a judge.