Because the alternative to not dividing by states is have a monolithic US, vastly larger than all other geographic units. Like, pick a threshold of size N. Sta

Because the alternative to not dividing by states is have a monolithic US, vastly larger than all other geographic units. Like, pick a threshold of size N. Start at the top, divide at countries, and the recursively keep dividing any given node until you have passed under the threshold N. California gets divided before Canada (and the UK).