While Intel’s 10nm was the canary in the coal mine, it has taken a couple years for the industry to fully grasp the sheer wall it has hit, and how the other foundries would hit it just the same. Cannon Lake’s extreme delay and Apple’s middling A10X and A11 single-threaded performance improvements (despite what it did with the latter's core) were leading indicators.
We're still getting shrinks, but they aren't timely enough to double transistor count every two years anymore.
While there are other areas that can be advanced, we really need materials breakthroughs to be able to push per-core performance again. Until then, we’re mostly stuck.