There were only two operating systems written in high level languages before Unix: MCP and MULTICS. Burroughs mainframes had a 48 bit word, and Honeywell mainframes had a 36 bit word. Burroughs and Honeywell were interested in selling those machines, not in spending considerable effort porting their systems to competitors' hardware with smaller word sizes. There was no business value in doing this.

Unix, which was very much influenced by MULTICS, initially ran on 16 bit machines. It's a lot easier to scale up (to run on Vax) than scale down, which is why Stallman based his GNU project around Unix rather than attempt to port ITS.