I recon -
Simply cost of testing vs benefit.
Max autoland capability single engine is cat 3A. Hence you can autoland it and then manually control the roll out. As such if you NEED to use full reverse, you can do so without any penalty on minimum RVR.
If you don't need to use full reverse then you can leave in the autopilot for the roll out.
As there is no operational benefit to Airbus certifying the rollout with full reverse why would they? Hence the limitation which they probably did test with the A320.
***Only my best guess though!***