is there a logical anatomical reason for kendo or other forms of sword-fighting imitating contemporary japanesse martial arts that the position of hands is thought this way... left power and right steering?
Answer: The both hands are equally important. There are two reasons that we need to emphasize the left hand as the power hand.
1. Right handed: Most of people are right handed so their tend to use their right hand more than the left hand. When that happens, you basically cannot do kendo.
2. The position of the hands: Since the right hand is far from the body, it tends to take over the control of the sword, especially when the handle is longer. When the left hand is away from the body, the left hand will take over. So it is a matter of how far your hands are from your body.
It is more like "to use the sword more effectively, we need to balance out the power of our hands/arms". To do so, the teachings tell us to "use less power in the right hand and more power in your left hand".
And of course, you need to use both hands to change the direction of the sword.
Sometimes the teachings need to be explained more clearly or need to be found out why through training.