>airspeed is generally measured in either knots or Mach
boatspeed is also measure in knots... but only very rarely in Mach (and not even primarily because the speed of sound in water is substantially higher than in air)
Historically, I'm fairly certain airspeed was measured in knots because of boatspeed. In software terms, the problem of navigating over the Earth's surface was forked from boats to planes, and why re-invent the wheel?
It also tends to be more of a standard in professional aviation, whereas some small civilian bugsmashers in the US use MPH, presumably because it's easier for the doctors and lawyers of the world to understand after driving their cars, and if you're VFR, who really cares?
boatspeed is also measure in knots... but only very rarely in Mach (and not even primarily because the speed of sound in water is substantially higher than in air)