Believe it or not, AIX is still big in some Fortune 500 countries that can afford the license and support fees.
But does AIX being a certified UNIX play a part in this or is it just because of IBM's reputation? If IBM stopped certifying their OSes as UNIX, wouldn't they still get the same contracts?
The latter of course, if anyone writes a set of requirements that lists "certified UNIX" they're trying to stack the requirements in the favor of a specific vendor. Although at this point I'm not sure who that would be since I'm not even sure who has actual UNIX certified OSes.
That would be IBM, HP, and Apple and probably a few more.
But have they kept up with it? I believe you have to certify each release of your OS and at this point that would seem like a waste of effort on their part.