I've been using a similar explanation for why TL12 pop 8 Regina is the capital of the Duchy of Regina and not TL13 pop 9 Efate. Regina got in early, before Efate overtook it in tech and population.It seems that a reasonable "in universe" argument could have been made that in some cases, a given world at one time had had a higher importance, but had since declined somewhat, and moving a capital world to another with a current higher importance was just simply an unjustifiable hassle.
Hans