It’s really not, x86 (CISC) CPUs could be just as efficient as arm (RISC) CPUs since instruction sets (despite popular consensus) don’t really influence performance or efficiency. It’s just that the x86 CPU oligopoly had little interest in producing power efficient CPUs while arm chip manufacturers were mostly making chips for phones and embedded devices making them focus on power efficiency instead of relentlessly maximizing performance. I expect the next few generations of intel and AMD x86 based laptop CPUs to approach the power efficiency Apple and Qualcomm have to offer.