Why? because that's when famine and pestulance are most likely to happen. That's when injuries, trauma, infections and disentary can be treated on mass. Infectious diseases have a habit of spreading quickly through troops living in close proximity to each other.
Knowledge about womens health, pediatrics and degenerative illness is more likely to be advanced during times of great wealth and prosperity.
Chinese village doctors were often part of a "big family," which meant that they were treating most of the descendants of known lineages. Because of this they were likely to see all the hereditary expressions of a particular genetic line. This made it possible to accumulate great knowledge about weaknesses and diseases which are inherited.
While medical knowledge in China was often past down father to son, and too often was secret. There is also a long tradition of publishing best practices.
Starting in the Tang Dynasty (600-900 CE), Chinese governments gave exams, and officially certified competent doctors.
The two dominant metaphors of Chinese medicine are the circulation system and the digestive system.
Circulation is associated with the North and acupuncture. It conceives of health in terms of a complex plumbing system, clogs, narrowing, pooling, not enough pressure, etc....
Digestion is associated with the South and herbal formulas. It conceives of all parts of the body in terms of digestive function, assimilation, elimination, appetite, fermentation, purification, etc....
A third metaphor, which tends to be associated with martial arts, is a well integrated structure. It conceives of every part of the body as having it's own optimal shape and way of moving. Each part contributes to a complete, well integrated whole moving form. If one part of the body is not moving the way it is supposed to, it will effect all other parts of the the structure. For instance, a liver that doesn't move like a jellyfish when active, will slowly, over years, change the shape and alignment of the bones. An injury to the neck will effect the dexterity of the hand.