FWIW, I read the other day in a Construction textbook that the reason why the US hasn't switched is because a system for integrating it into construction standards did not exist.
But, that one was being designed and was almost ready to be distributed, and that once that happend, the switch all around would happen.
Conversion for construction is much more complicated than it appears.
Everything in the National Building Code used to be based on 4 inch modules -- e.g. 16" between studs, 48" X 96" plywood, etc. A "hard conversion" uses 10 cm. modules -- close but not the same. Metric plywood is slightly smaller at 1200 mm X 2400 mm (everything is measured in millimeters, to avoid any need for decimals or fractions). Thicknesses are different too.
Most house builders still use Imperial (I think). The two don't mix. If you use metric plans, you need metric materials -- soft conversions simply aren't practical. (Too many decimals.)
You can get by without a metric hammer though. :-)