Moore's Law is struggling against the limits of what's physically possible and the days of massive jumps in performance each generation are long over. It's hard to advertise a new CPU when it's running at nearly the same clock speed models ran at ten years ago and all you've done is thrown more L3 cache at an otherwise 99% unchanged previous model.

Even the number of cores has stagnated, and that was their Hail Mary to keep the performance gains coming. And I bet the Spectre/Meltdown mitigations eroded a large part of those gains too - IIRC existing chips lost ~12% performance after the firmware patches, and new ones had to be redesigned to handle speculative execution in a less efficient (but more secure) way.