mmHg is reasonably common because that's how the first pressure gauges worked, and measuring how much a pressurized gas or liquid can move mercury against gravi

mmHg is reasonably common because that's how the first pressure gauges worked, and measuring how much a pressurized gas or liquid can move mercury against gravity is easy and reliable enough that this style stayed in use for 300 years [1]. But 0.8mmHg isn't a very impressive number, so at some point someone got the bright idea to use mmH₂O instead. [1] https://en.wikipedia.org/wiki/Mercury_pressure_gauge https://en.wikipedia.org/wiki/Mercury_pressure_gauge