In the book
Measuring America by Andro Linklater, the author points out that some Imperial/Us Customary units are more convenient for everyday activities than their metric equivalents. In routine day-to-day transactions people prefer to have units that provide roughly whole-number quantities. He points out than in Europe, meat is sold in the 500 gram "pfund"/"livre," etc., coal and lumber in Germany is sold by 50 kg "Zentner" or "hundredweight," and Swedish and German plumbers use "Zoll" or "inches." (and they aren't even based on any known metric unit.)
Another thing he points out is that converting to metric in the US would be really messy, as every plot of land in the US has been surveyed in feet/rods/chains/acres, etc. Apparently the surveyor's chain, based on multiples of 4 (instead of the multiples of 10 used in the metric system.) is very versatile for measuring and subdividing land (the better to buy and sell it), an activity that Americans seems to have a near mystical attachment. I'm not sure how the Canadians, Brits, Australians, etc. handle it, but I suspect that even if we went metric tomorrow, land deeds would still have to be measured by traditional means for a long time to come.