Moore's law described transistor counts doubling roughly every two years, and single-core clock speeds stopped rising around 2005 while transistor counts kept climbing. Why did the two part company?

A. Because power density became the limit, so transistors went into more cores.

Power scales roughly with frequency times voltage squared, and higher clocks needed higher voltages, so heat rose far faster than performance and the wall arrived while transistor budgets kept growing. The extra transistors went sideways into cores, which made it a software event: a sequential program stopped getting faster by waiting, and Amdahl's law sets the ceiling.