Given king title is not fixed to particular NPCs like in the Warband anymore, how would you implement that? In Warband only some NPCs were or could become kings and they were immortal.
True. Plus most noble men and women have very high, if not the highest, gear their culture offers (Garios in imperial hauberk and scale skirt, Ira in scale armour etc.). The same is true for Warband, by the way.
I think its a bit strange that pretty much every faction ruler in Bannerlord just wears what every other lord is wearing.
Well, if you activate the cheat mode and have a look at the high tier armours and helmets, you will notice that rulers wear the best gear and nobles the second best... It is not totally unique but it is the closest the life-death-birth-cycle allows to be done.
A historical note: Bannerlord simulates the late Roman "senate", the northern and Germanic "thing" and the Holy Roman "Kurfürsten". These three are ancient and medieval electoral systems because in the European middle ages
kings were
elected out of a pool of influential nobles. They wore the best armour they could affort "tailort for them" , but not unique in appearance (except for a crown mounted on the helmet, which can be found in Bannerlord too). Only in the modern era (1500 onwards) special armour for kings was a thing, because in the Holy Roman Empire the trend shifted from elections to heriditary systems of "kingmaking" which ended in absolutism since the 1420s. 1084 is not 1420 or even 1500.
Clothing on the other hand was always very special for Kings and Queens/faction rulers. This is were we can see unique items as well as weapons: unique clothing and decorative swords during public events were royal, but on the battlefield there was not much of "unique" armours or arming-swords (the weapons used for fighting, not for showing off)...
Sorry for the lecture.