Downward aileron travel must be restricted when flaps are deflected. Since flaps change the local incidence on the flapped part of the wing, the outer wing will experience an increase in its local angle of attack. The increased suction over the inner wing will accelerate not only the air flowing over the inner wing, but also that air which will flow over the outer wing as well. Left to itself, the outer wing would stall if powerful Fowler flaps are deployed on the inner wing.
Adding slats will push the stall angle of attack up, and now the outer wing is back in business. However, if the aileron is deflected downwards, the stall angle of attack is reduced again. The outer wing could stall simply due to the aileron deflection!
I do not need to point out that an asymmetric stall on approach is extremely undesirable.
Now roll control could still be achieved by only moving one aileron upwards. But this would decrease the induced drag at the wingtip at low speed. Remember, at low speed the induced drag is dominant (it is proportional to the inverse of the flight speed squared), so the drag change due to an aileron deflection will cause an inverse yawing moment at a time when the pilot wants to keep the aircraft lined up with the runway.
Using the spoilers instead will incur a higher lift loss for the same rolling moment, but will also increase local drag, which creates a helpful yawing moment into the opposite direction of the one created by upward aileron deflection. Also, the change in local lift from spoiler deflection is much greater once the flaps are fully deployed. This means that spoilers are much more effective for roll control in the landing configuration than in the take-off or cruise configurations.
THAT's why spoilers are preferred for roll control on approach. I am surprised, however, that you observed no aileron motion at all. Normally, the ailerons are still used, but with a much restricted travel range, as @DeltaLima mentions in his answer.