The reason is very simple. The CAG is a constitutional authority. IAAS Officers, while being Central Government employees are not under the administrative control of the Government while serving in their own department.
Except for the top 13 posts ( that is Deputy CAG and Addl. Dy CAG level) the CAG is entitled to create any posts to accommodate his officers. This means that irrespective of the intake of Group A officers into the service, enough number of posts are created to accomodate all of them at the higher levels.
This is in contrast to other central services like IRS where the Finance ministry has to approve the cadre review. This results in stagnation. IAAS is able to promote its officers because its cadre review lies in its own hands. This power can’t be arrogated by the Ministry of Finance and hence you can always be assured of timely promotions in IAAS.