Why does the holder of the Iron Throne not have a CB on the Iron Isles?

[blackninja9939]

Maybe because they don't normally belong to the seven kingdoms. Wouldn't it be "Eight Kingdoms" if they were part of it?

I think you are misunderstanding the concept of the Seven Kingdoms...
The Seven Kingdoms is the seven independent Kingdoms at the time of Aegon's Conquest: The North, The Iron Isles, The Vale, The Rock, The Reach and Dorne. The Riverlands is not counted as one of the Seven Kingdoms.