The first reason is that Jin has many soldiers, while Chu is often short of soldiers. After all, the state of Jin has already formed a scale and the population is very dense. When the king of Jin wants to recruit soldiers, just put up a few notices casually. At that time, although the territory of Chu was large, it belonged to the kind of vast territory and sparsely populated, and its population was far less than that of Jin State. Therefore, the king of Chu often failed to recruit soldiers. In that era of cold weapons, the number of troops was often the key to determining the outcome of the war.
The second reason is that Jin's culture is prosperous and talented people come forth in large numbers, while Chu is very backward and lacks talents. In fact, fighting is basically about the cultural literacy of the two countries. The cultural development of the state of Jin not only produced many military talents and politicians, but also had excellent material bases with high technology such as weapons, armor and medical care. Chu is not only short of talents, but also lags behind Jin in weapons and armor.
The third reason is that Chu's diplomatic strategy is not good, and neighboring countries don't want to see Chu's strength. After all, the powerful state of Jin at that time gave many threats to the surrounding small countries. If there is a powerful Chu state in the south, those small countries will be squeezed between two powerful countries and will be annexed by one of them sooner or later. So they would rather be bullied by the State of Jin than by Jin Chu. Their destruction of Chu is also an important factor in Chu's repeated defeats of Jin.