1. Power efficient design is a priority
Designing for performance without caring about anything else can't happen anymore. CPUs already run at 130W or more. In the days of rapid performance increases(for both CPU and GPU), the power usage were really low.
Only a little over a decade ago, Geforce 2's consumed less than 30W and Pentium 4's going over 50W was a big surprise. Moore's Law allows power reductions, but designs have gone way above that each and every generation. Claims made by companies like Nvidia saying their GPUs are advancing at 3x the speed of Moore's Law wasn't being done for free. It wasn't going to last.
Nowadays we say we care about "power reductions" but in the Pentium MMX days the HSF combos were barely larger than what we use in chipsets. Few years before that, active cooling weren't even needed.
2. Workload benefits are plateauing, or becoming really difficult to optimize
Back in 2005 when first dual cores were surfacing, the benefits were throughout user and application base. We dreamed of multi-core era lasting forever and single thread IPC gains seemed no longer important. That's not the case anymore. When we moved to quad cores, it took significantly longer to take advantage of its gains. With 6 cores, its even less than that.
3. Physical limitations?
In the Athlon and Pentium III days, the war between two companies AMD and Intel were basically all about clock speed. Then few years later, Intel predicted 10-20GHz computers in 2015+.
Nowadays the record for highest speed is only set in overclocking. But not even with liquid nitrogen cooling, voltages unimaginable to mere mortals, with cherry-picked parts can we reach 10GHz. It makes me wonder if its even possible to see 5GHz chips at stock at all.
That does kind of suggest they can't utilise them to improve the performance of the CPUs calculation or its no longer the priority.
Remember what Anand said on his reviews? It used to be CPU architects did quite a bit to get that extra 1% in performance. But with newer architectures, the circuits are only used if 2% performance gains happen with only 1% increase in power use.