The Smithsonian is an American institution and as such caters to American's habit of using Imperial measurements. This makes it more relatable to the the passing reader.
Same here. (Although, I've driven a couple cars that only had a single scale of digits on the gauge. Pressing a button would toggle the meaning of the numbers from mph to km/h and swing the needle around to point at whatever was correct.)
The other issue relates to the fact that we build things according to imperial dimensions. (Paper is 8.5x11, bolts have sizes in imperial units, screw threads are pitched according to inches, etc.)
Some or all of this can be changed, but neither the change nor the transition are free. (ie: I two sets of tools - one for metric and another for imperial.)