I would suspect that the reason you can't do a CATIIIB approach with a failure case - which is what I think that message is telling you, that you're down to a single channel system, no redundancy - is that IIIB requires a fail-operational status i.e. that everything keeps working after a failure. Obviously if you're on a single-string system it can't be fail-operational.
Suppose your single system goes offline below the IIIA DH, and you're operating as per IIIB rules. You aren't assured of the GA, but have no assured landing either. To avoid that problem, you have to respect IIIA rules.
Caveat: supposition, not knowledge of the Airbus system.