It also has to do with the lore of each of the starting races. For example, Kojani and Half elves (which are half kojani) share the large urban center, which makes sense. The Goblins start the game as slaves to the orcs, so it would be logical they start in the same spot. and wood elves and raki, both being elusive tree hugging forest dwellers would naturally go together as well. You have alot of alliance/interdependence on Kojan.
Thestra however, is populated by lots of people who hate on each other. Only the barbarians and lesser giants (ironically) seem to have some sort of kinship. Vulmane and elves are mortal enemies. Thestrans (the white man) are too busy being humans to care about other races. Dwarves are naturally standoffish and isolationistic. Halflings, while they get along with humans, are often treated poorly by the general human population because of the stature and demeanor. although, before the undead invasion they basically lived across the river from the humans in old traganor (sp).
Also, from what i understand the starting areas quickly filter the races of thestra together. Barbarians, lesser giants, and vulmane have parallel content as soon as lvl 10-15 iirc. Halflings end up mixing with the thestrans very quickly, and i bet dwarves and elves come into that mix fairly early as well. By lvl 20 everyone is all over the world anyway. I know if my homies start on Kojan and I on Thestra, I will be with them on Kojan by the end of my first gaming session.