>Although the ARMv8-M architecture is 32-bit, it also supports data types of various sizes such as 8-bit and 16-bit. ARMv8-M also supports a limited set of 64-bit operations.
It looks like it still uses the 'Thumb' instruction set, which combines 16- and 32-bit instructions in an attempt to balance code size and operation complexity.
This is for microcontrollers so it doesn’t need 64 bit. I don’t think silicon vendors want 64 bit since making it 64 bit requires more gates. Cost matters.
