Why?
I'll use the example of money.
How come Canadians have a 1 and 2 dollar coin and whenever the US tries something it fails? The US never just says "on this date, the paper bill is no more and you'll be using coins now, tough love."
No. The money issue is worlds apart from this issue....it's so far removed, that it isn't even in the same univers.
In the lumber industry, the spin off effect would be monumental. Blueprints, massively expensive equipment in the billions of dollars, that are calibrated for imperial. Fixtures in plumbing, electrical, home power tools....industry power tools. This list is endless....not to mention a ton and a half of stuff that I'm certain that I'm not aware of.