u might asked urself with such a question: high attitude compared to what?
if u compare it to a straight wing, i guess the answer could be: due to angle at which the wind hits the wing (lateral component, not the vertical one) u got a lesser windspeed than when it hits the wing in a straight manner (as in the case of a straight wing). To compensate for this relative lift loss u have to increase the attitude (if CAS remains the same in both cases).