I believe the main reason is CPU micro-architecture (note: I have had way too much exposure in my life to the design of the CPU in your phone and in your laptop

I believe the main reason is CPU micro-architecture (note: I have had way too much exposure in my life to the design of the CPU in your phone and in your laptop to have a non-biased opinion) What does that mean? Essentially that the race for deep pipelines has ended, with 20-40 stages being the optimal depth. After that, miss penalties just hurt too much. Therefore, when you can't make the pipeline deeper, you can't make the frequency much faster, you are stuck with following process progress (which is already pretty good). So it's more tempting to go after multi-cores: same pipeline depth, more silicium, more efficient overall.