It's also surprising that Wisconsin and Minnesota do well given their winters. Perhaps the hardness of the winters forces runners to do base training. When I lived in MN, we ran all winter and there was one guy who wouldn't where a t-shirt until it was 32 degrees.
I think the change of seasons is very good for runners. In the colder weather, your body is able to train faster (its not encumbered by heat induced diminished performance). During the warmer seasons your body learns to perform and acclimates to warm temperatures. Heat dissipation improves.
In CNY the hilly terrain may be an additional positive factor.