I read that the out of order execution of the RISC was simpler to handle with the fixed 32 bit instructions. They said Apple managed to dispatch 8 instructions in parallel whereas the hi end CISC (x86) tops out at 4.
The greater simplicity of ARMv8 and its fixed sized instructions definitely helps, but also Intel runs their cores at nearly 2x higher frequency, which means a lot less logic can be squeezed into a clock cycle. That makes it much harder to to make a wider processor.