The reason is when the ball makes contact with your paddle, no matter how strong you are, your paddle head flexes back slightly. Therefore, if your paddle head is above your wrist and the paddle flexes back, your paddle head is facing up. Therefore, the trajectory of the ball will be up and over the net.
If your paddle head is below your wrist, when it flexes back, guess what… your paddle head is facing down or straight at the net. Therefore, the trajectory of the ball now will be directly into the net.