Kusari is not getting invaded because the dev team does not have houses get invaded for the lolz.
Funny how that works.
Why should they get invaded? It's not like their activity is completely dead like Bretonia's was in 4.84, especially if 4.86 opens nifty trade routes between Kusari and the Gallic Core Worlds. There's no pressing need to rewrite the story to throw activity at Kusari, so why not go with what makes strategic sense and presents more storytelling options later on down the road? It's anyone's guess as to whether Kusari will re-enter the war at a later date, and if they do they could feasibly enter on either side. This way has more options, and Bretonia gets super-heroic to boot. Why should we abandon that just because Kusari hasn't had overwhelming military force applied to it since vanilla? Yes, remember when they got invaded by Rheinland, got their asses handed to them (due in no small part to Nomad-based hax), and then started to get invaded by Liberty before the Order wrapped things up in Omicron Major? If Kusari did get invaded, I think you'd probably be yelling at us that we hated Kusari for steamrolling over them for the second time in two decades.