Jason Snell, in his More Color column for Macworld:
And that strikes at the heart of it: The Mac will continue to grow and evolve, but it will do so within some very specific constraints. Meanwhile, the iPhone drives the bulk of Apple’s revenues and will do so for the foreseeable future–and that’s Apple’s biggest priority. The Mac gets a lot of benefit from riding along with Intel’s chip developments for personal computers, and it saves Apple from having to divert its attention from iOS in order to invest more resources in a small portion of its business.
Now, watch: Apple will announce an ARM-based MacBook in the spring of 2017. It’s not impossible, not at all. But I’d bet against it. Apple’s got too much riding on the growth and improvement of the iPhone to put its stable platform through an upheaval.
I remain bullish in my prediction that Apple will release an ARM-based Mac sooner than later, but Jason's logic against it is sound.