Because of Newtons second law of motion, which says that the force exerted by a body is directly proportional to the rate of change of momentum of the body.

  Here the body is the ball. Before hitting the cricketer hand the ball was travelling with speed v and after hitting his hand, if the speed reduces to zero in very less time, then rate of change of momentum will be very high. Thus , the force exerted on the hand will also be very high. Therefore, the cricketer lowers his hand, thus reducing the speed of the ball slowly to zero instead of immediately reducing it to zero. This decreases the rate of change of momentum and hence the force exerted on his hand is also decreased, thus it saves his hand from getting hurt.

Force=m(v-u)/t

v->final speed

u->initial speed