POWER and Power (formerly PowerPC) are similar but quite different. PPC has been in embedded (but generally not mobile) for quite a long time, but even then, the cores are still hot, power hungry, and poorly suited for mobile.

Because the designs predate the big push into extremely high efficiency processors. Like the big "server class" processors, the investment required to create a truly high efficiency processor is quite large. Small in-order cores with limited functional units, and lacking much of what makes a modern processor fast (vector units, specialty instructions/etc) can fool people into thinking that minimal clock domains/gating is sufficient to create a high efficiency design.