ARM didn't "come out of nowhere", it's been used in embedded and mobile systems as a low power CPU for decades now. The earliest mobile use I can find is the Apple Newton in 1993. DEC was marketing their StrongARM product as a low power CPU back in 1996.
And it goes even further back than that: when Acorn got first silicon back for the ARM1, they were hoping for anything less than 1W so that affordable plastic packaging would be sufficient. When they booted up that processor, their multimeter on the power supply line didn't register anything. Further investigation showed that the supply line wasn't actually connected, and the processor had booted using a total of about a tenth of a watt of leakage from other components on the board.
Intel was almost dumb enough to put those on the used manufacturing line from x86 chips... Almost. Then Intel "knifed the baby" by moving those designers to Atom and selling off their ARM stake... At one point StrongARM was the top of the hobbiest Boards available, long before Raspberry got their idea.
