As I understand it most Linux distros adhere to the UNIX specifications while only a few actually get UNIX certified. If plenty of Linux distros can happily adhere to the standard without being certified in it, what then motivated Apple to get MacOS UNIX certified?
According to Terry Lambert, the engineer who led the project to make MacOS comply with the Single UNIX Specification,
[...] it was done to get Apple out of a $200M lawsuit filed by The Open Group, for use of the UNIX™ trademark in advertising.
So the systems could be sold into organizations that happened to have a "must be Unix certified" on their checklist of requirements.
Who would have such a requirement? Who knows. Certainly much less germane today, but back when it was introduced, the workstation wars were still going strong.
The assorted Linux systems don't have the certification because they never went through the process and cost of doing it. Apple could do that, so, check that box and move on.
I do not know the exact answer (so if this is too much speculation, feel free to close as subjective), but I do remember the situation Apple was back in at the time, and my theory is "Because they desperately needed the marketing boost".
Mac OS X started as a push into the Server market as well - the very first version you could buy was Mac OS X Server 1.0 which even lacked the Aqua user interface and still looked like Mac OS Classic.
Combined with the fact that Mac OS X's XNU kernel is based on a "real" unix (4.3BSD), it seems like a logical choice (and low hanging fruit) to get the "UNIX Certified" badge to show that they are serious about things - in the corporate world, you couldn't get much more serious in the late 90's than "UNIX".
I don't know if there were markets where the certification mattered for compliance or if they just wanted to impress decision-makers (e.g., Apple's a huge player in the American education market ever since the Apple II), but I almost see more reasons for than against getting UNIX certified at this point for them.
That said, this is my speculation. I do know that Apple had a pretty rough reputation in the late 90s (Michael Dell's "Sell the company and return value to the shareholders" quote is legendary for how it aged like milk, but wasn't all that uncommon of a thought at the time) and if you listen to the Boston 1997 Keynote you can clearly see a just-returned Steve Jobs trying to find ways to rebuild the brand - and what better way to tell companies you're serious by making a serious UNIX? The 2002 Xserve event shows a very humble Steve Jobs really needing to convince people to trust Apple.
Also, Mac OS Classic had a reputation for bad system stability - it was possible for an application to crash in a way that destabilizes the entire system, which was acceptable in the 80's and early 90's, but for people used to Windows NT and UNIX, it was becoming less and less acceptable. That said, the Mac OS-Classic based iMac was an absolutely massive success, so I don't think that the consumer market mattered, even with the NT-based Windows XP now moving consumer-Windows into the modern age.