VikS, I think it is because the virtual stick forces are too high for the virtual pilot to be able to pull back the stick far enough to exceed the critial angle of attack. In other words, the virtual pilot is too weak to overcome the stick forces enough to stall at high speed. If you use lots of positive trim, you may be able to do it.

This is a problem fairly universal to simulators, not just IL-2, and is a result of trim being impossible to model correctly due to the greatly different nature of real aircraft sticks and gaming joysticks, and due to the necessity of a virtual pilot. I am glad that this will not be a problem with Rise of Flight, since Great War aircraft didn't have trim controls.