Why isn't GNU/Linux SUS v3+ compliant?
To get a certification you need to pay, and it's actually really expensive. That's why BSD-like and GNU/Linux OS vendors don't apply for it.
So there isn't even a reason to check whether GNU/Linux is compliant or not.
Most of all, the GNU/Linux distribution follows the Linux Standard Base, which is free of charge, and recognized by almost all Linux vendors.
Edit: As my answer is not completely correct, I'll add the @vonbrand comments:
Linus (and people involved in the development of other parts of Linux distributions) follow the pragmatic guideline to make it as close to POSIX as is worthwhile. There are parts of POSIX (like the (in)famous STREAMS) that are ill-conceived, impossible to implement efficiently, or just codification of historic relics that should be replaced by something better.
... therefore, does it make it harder to obtain a certification?
Sure. POSIX mandates some interface, which Linux just won't ever have. Case closed.
First it's simply because there's little incentive doing that. Users don't care whether a distro is Unix-certified or not. As long as it fits their purposes they'll use it. Moreover the certification costs money, and getting it doesn't make sure that the distro will get more money for development and maintenance
However if you look at Single UNIX Specification's Currently Registered UNIX systems or POSIX-certified systems you'll see 2 Linux distros in the list
- Inspur's K-UX
- Huawei's EulerOS
The Open Group official register of UNIX Certified Products also confirms that they conform to UNIX 03 Product Standard A.K.A. SUS v3
Is there a Linux distro that's UNIX certified?