Because you roll laterally around the longitudinal axis, and you pitch longitudinally about the lateral axis. The axis about which you move is at right angles to the direction of movement. If you move your arm out sideways, it moves about an axis fore/aft through your shoulder, whilst your arm moves sideways. The longitudinal axis of a plane will always go from nose to tail- the only way you can move around this axis is laterally, ie roll. How can you change this?