Actually power efficiency was a side effect of having a straightforward design in the first ARM processor.
The BBC needed a cheap (but powerful) processor for the Acort computer and a RISC chip was
When ARM started testing their processor, they found out it draw very little power...
Acorn won the bid to make the original BBC home computer, with a 6502-based design.
Acorn later designed their own 32-bit chip, the ARM, to try to leapfrog their competitors who were moving to the 68000 or 386, and later spun off ARM as a separate company.
