Beacause
AFAIK UAS is being kept
by the System as an "INSIDER INFORMATION". A privilege of the System. (as per
mentioned Airbus SAS paper crew must scan...)
IMO this is posted before by several posters:
Crew must scan because system has limited pre-programmed parameters to make logic decision to decide which ADRs are wrong (if dual or more ADRs are wrong and different).
In such a case a single good source can be rejected by the system.
Human (and Crew in specific) can add or skip parameters to make a - better! - logic decision on wrong and different information.
They have to isolate the faulty sources to prevent the system for rejecting the only good source and using the wrong sources.
Nothing to do with 'INSIDER INFORMATION', unfortunate during the fase 1 (as determind by BEA) the UAS of 2 or more sources lasted not long enough to trigger the ADR DISAGREE ECAM message (needs 10s to prevent spurious warnings).
in fase 2 (Continued Stall warning fase) all the speeds returned to consistent values.
In the start of fase 3 - when AoA became invalid due to hi AoA (resulting in CAS NCD) the ADR DISAGREE triggered because the UAS lasted more than 10s.