It's the seven and thirteen thing. In the original myth it was the X-blade that split into 20 pieces, 7 of light and 13 of darkness. So, when the children rebuilt the worlds, there were seven pure lights born into it. This is also why there need to be 7 GoL, because Xehanort wants to reforge the X-blade and he needs 7 and 13 again, so if we don't raise the Guardians he'll just use the princesses.
I think they're labeled "princesses" because they're all young women and it just so happens that most of them are from royalty.
I also like to think that as current PoH grow old ans pass away, new ones are born, ala the Avatar cycle, except there's seven of them at a time and the world they are born into is random. The opposing theory would be that it's always been these same seven and up until now, time in their worlds has been moving astronomically slower than it is now, which is really saying something considering Cinderella doesn't seem to have aged a day in the 10 years since BbS. Also Kairi, the seventh PoH, was only born 4 years before BbS and there's no way the Keyblade War ended that recently even within their own time dilation.
