This is determined by the military organization, strategy and tactics, ideological quality and temperament spirit formed by the specific historical background and specific weapons and equipment.
If the quality of the generals of the two sides is similar and there is no big difference in military literacy, the result will be like this. There will never be a situation in which modern troops can conquer the world.
For example, there were a large number of cavalry in the cold weapon era, but the modern army was basically eliminated. The tactics were close-range slashing, which was eliminated by the modern army, and it was difficult for inexperienced troops to adapt. After all, the visual effect and psychological impact of killing an opponent from a distance are very different from cutting off his head or parts in front of his eyes.
The modern army has adapted to the convenient communication, changed the ancient equipment, and failed to achieve rapid contact, which will inevitably lead to ineffective command.
It should be said that the modern army and the ancient army have their own advantages and disadvantages.
Shorter feet and longer inches.