Because of Newton's second law of motion. Before hitting the cricketerâ€™s hand, the ball was travelling with a speed "v" and after hitting his hand, since the speed reduces to zero in very less time, so the rate of change of momentum that is the force is very large. That is why the cricketer used to lowers his hand. By doing this, he reduces the speed of the ball slowly to zero instead of immediately reducing it to zero. This gives him a few additional microseconds and the rate of change of momentum decreases.