Sounds like a lack of external contributors maintaining it to me, are there really that many users? Everyone I know on macOS uses docker (or some other virtualisation) to run linux for small jobs and then connects remotely to linux boxes when they need more computing power.
Unofficially, there may be some people using Hackitoshs with rather beefy GPUs for machine learning.
There's a lot you can do easily on a $500 GPU that should take too long on CPU. And I prefer the shorter write/run/debug loop of working locally. It's the same niche other machine learning workstations fill, only with the preferred desktop OS.
There will also be external GPUs for Macs soon(ish), and those would be perfect for tensorflow. I'm not sure at that point they'll want it running on Macs again, and discontinuing support now may be the wrong decision.
http://www.everymac.com/systems/apple/macbook_pro/specs/macb...
I have a MacBook Pro from 2013 and I think it was the one before the last to have a Nvidia GPU.
Also, it's always an headache for me to get CUDA to work with Python/R libraries anytime I update the system and I have yet to find a straightforward guide that tells me exactly the steps to take... they all fail at some place.