Just to throw my 2d into the ring...
Remember the old formula L= Co-efficient of Lift x 1/2 p x Vsquared x S
As p (the Greek letter rho representing air density) decreases with increased altitude, lift decreases. To maintain the same amount of lift you have to increase the Co-efficient of Lift or V (IAS) or S (the surface area of the wing). Increased IAS means more power ie. fuel burn. Increased S means something akin to Flowler flaps that would be very complicated. That leaves Co-efficient of Lift. Co-efficient of Lift can be increased by increasing the AoA. Hence, at higher altitude an aircraft needs to maintain a higher nose attitude.
Incidently, 4 degrees is generally recognised as roughly the most efficient AoA.
Why they don't just increase the Angle of Incidence (the angle at which the wings are attached to the fusilage), I don't know. For that, you'll have to ask an expert.
Hope this helps and I welcome any comments to correct any erroneous statements.
Cheers, LP
