The Kandrian Empire was large. Especially when compared to the many small sovereign states in its immediate geographic neighborhood.
Most of these small states lacked a strong Martial foundation. They were unable to sustain or retain many of the Martial Artists that did end up breaking through from their populations.
The reason was brutally simple; Large nations were more attractive to Martial Artists than smaller nations. Many of them moved to the larger nations such as the Kandrian Empire, the Sekigahara Confederate, the Britannian Empire and the Republic of Gorteau.
These nations were able to facilitate their growth as Martial Artists to a much higher degree than smaller nations could. Due to a greater availability of better learning and growth resources.
