Theres alot of things in Bleach that havn't been explained. What the Vaizard are, the Espada, the Arrancar and their goals and why certain families in soul society are more famous than others.

My guess is probably because there more powerful in fighting and politically maybe. The Kuchki family would be a good example. Then I again im just guessing.

The More I think about it the more it seems that Bleach is goanna be a long long story line. Cause you know all the captains and soul reapers, Vaizard will all play apart in the war as the Squad 1 Captain leader said.

I just wish the story would just move a little faster.