ARM was designed because the 6502 was approaching end of viability, and Acorn (the maker of the BBC Microcomputer) needed a next-gen product. At the time, RISC was the trendy thing, and I suspect the 286 and 68000 were too expensive to adapt for their products; they weren’t pushing £5000+ workstations like IBM or Unix vendors.

It was light and small because they had a small team; low power was a happy accident.