I think it's a matter of convenience. One inch is a handy unit for movement and ranges. Only in rare cases you want to use anything smaller than that. Typical moves are somewhere between 3 and 10 inches, which are nice, small numbers. With cm:s, most of your moves would be over ten and many ranges in hundreds.
As for miniature sizes, mm:s give similarly handy integer values. One mm can matter, sometimes, but half doesn't. Conversely, 1.10236 inches or 1 and 13/127 inches wouldn't be that handy.
However, while miniature sizes are essentially just names and rarely mentioned in the rules, it gets odd when all the base sizes are in mm:s. You have 20mm bases moving four inches. It happens in many other games too. Don't they know that it can
destroy $200 million Mars orbiters?
