Why does the A320’s maximum allowable aileron deflection decrease, rather than increase, when the flaps are extended?
You assume that


  the lower attack angle of the unflapped, aileron-containing portion of the wing should allow the ailerons to deflect further without stalling.


but that only looks at the geometric angle of attack. What counts, however, is the induced angle of attack. Since the flaps on the inner wing create much more lift, the induced angle of attack is much higher than for the clean configuration at the same geometric angle of attack. Consequently, the outer wing will stall at a much lower geometric angle of attack, and adding trailing-edge-down aileron deflection will stall the outer wing with only a medium angle.

Add to this that the spoilers are much more effective for creating rolling moments once the flaps are deployed and will avoid the adverse yaw of an aileron deflection due to their high drag on the down-going wing, and the spoilers are a much safer option for roll control that the ailerons are in landing configuration.