For the same reason that for example a 737 can dispatch with one pack inop only if the cruise altitude will be limited to FL250 but the QRH does not mention any maximum altitude in case of a failed pack - subsequent failures will potentially result in a loss of pressurisation. In flight a failure of the other CAC on the same side will result in a PACK L / R EICAS, and the checklist will tell you that pressurization may not be maintained 'near maximum operating altitude'.