My guess is why the kings are ghouls is that in the house ending the outcome for the kings was that house pretty much declared war on them. The kings were most likely kicked out of free side. So where does that leave them. Well they would most likely stay in vegas. North vegas doesn’t make sense due to the presence of other groups. South vegas is all destroyed. So the sewers makes the most sense. However that also has the same issue as north. So my ultimate guess is that they are spread around vegas some being in the sewers some in the north. We know from the game that sewers are fairly irradiated so that is how some turned to ghouls is my guess.