Is that because of technical issues or more related to the smaller marketshare?
Generally its technical issues - poor drivers, frequent and disruptive OS updates, and Apple being Apple. More than one SW vendor has repeatedly said the OS X version is the most troublesome of the three.
I can only imagine the market share is healthy enough at the moment to put up with it. New Apps are still arriving on the Mac platform so I guess times are still good.
One Red Flag is in the form of GPU renderer RedShift. I could have sworn they used to say 'OSX version coming soon' but no more. It's CUDA only for now, and so thanks to Apple they suddenly have virtually no market on OSX. Hope they hadn't spent any money on it. Another CUDA-only renderer, Octane, still lives afaik, but Octane for Nuke has been put on hold. The developer's own words...
'Whilst the plugin is very stable on Windows and Linux, I have had a lot of problems getting it to compile and run in a stable fashion on OSX. The Nuke OSX plugin development environment is challenging to work with - so I have stopped work on the OSX version. If there is sufficient interest from users, I can take another look in the future.'
Edit: I'm forgetting another GPU renderer, VRay RT. This from 2013 no less...(Ars)...
"Apple may be trying to not-so-delicately nudge everyone to move their code from CUDA to OpenCL, but I’ve seen a first-hand failure of AMD’s OpenCL support with V-Ray RT for Maya. Chaos Group built V-Ray RT on OpenCL, but after extensive work trying to get the GPU variant of its RT render engine running on AMD hardware and an effort by yours truly to light a fire under Apple and AMD, Chaos Group gave up and ported it to CUDA instead. So V-Ray RT’s GPU mode only works with OpenCL and CUDA—on Nvidia hardware.
If Apple wants OpenCL and AMD to be the answer to CUDA, the support needs to be there so this kind of thing doesn’t happen again. As it is, I now have zero options for V-Ray RT GPU on the 2013 Mac Pro since the software doesn’t work on AMD cards, and no Quadro/Geforce card is available for the machine. I’m sure some company will eventually build a Thunderbolt 2 Nvidia GPU-in-a-box for people who need CUDA, but to quote our own Peter Bright, “Thunderbolt 2 is equivalent to 2.5 lanes of PCIe 3 or 5 lanes of PCIe 2,” so it's hard to say what kind of a performance hit that will incur for compute tasks with an external GPU."
Nothing this author wrote improved. Afaik, VRay RT still doesn't work on Mac/AMD combos. eGPU's didn't show. And even worse, it looks like Apple's attempt to destroy CUDA in favour of OpenCL was just the prelude to destroying OpenCL. As for why? I've no idea. Vendor lock-in is a good conspiracy theory, but which vendors? Games? VR? GPU renderers? There isn't going to be anyone left to lock in.