Mostly the instruction set architecture. In the early 80's when Acorn Risc Machines developed ARM, it was part of a then-newer "Reduced Instruction Set Archite

Mostly the instruction set architecture. In the early 80's when Acorn Risc Machines developed ARM, it was part of a then-newer "Reduced Instruction Set Architecture" school of thought that had the following goals: - make instructions and CPUs simpler for the CPU and not human beings, since most assembly is created by compilers and not humans. - use a "load-store" paradigm that preferred registers over slower RAM for intermediate uses, - use the chip space saved by simpler CPU instructions for more registers to support #2 above. x86 traces its lineage to the early 8080 CPU developed in the 70's. This is when RAM was as fast or slower than the CPU, when 4k of RAM was $2000, and hand-assembly was more common. So it made sense to optimize for the most action out of each instruction and rely on memory for intermediate uses. The RISC/CISC "war" was before the early-to-mid 90's brought multi-core CPUs, large CPU caches, SIMD instructions, and numerous things CPUs do to the instruction stream like branch prediction, register renaming, out-of-order execution and etc. that are common now. So it doesn't matter so much anymore. SIMD in particular is something like the RISC paradigm on steroids - load large, wide register files with values and do operations on them all with a few instructions. ARM's simplicity, along with the fact ARM licensed its designs out to other silicon designers/integrators, now translates into lower power usage/heat output and ease of integration into system-on-a-chip designs. Hence why it's super common in phones.