I vastly oversimplified my previous answer, not expecting you to be interested in details. It started in the 70's and never really finished. There are actually a lot of things that are still Imperial, such as square footage for real estate, 1/8"-1/4"-1/2" etc. and board-feet for wood or other construction materials, and a lot of others. If you want to see a very good treatment of the subject, go to Answers.com, enter 'metrication', and scroll down to 'Canada'.
I only learned metric in physics class, because I was out of school before it started nationally.
By the way, one Yank who rocketted off a curve on the #3 highway was heard to exclaim to the cops "You Canadians are
nuts having a hundred-mile-an-hour speed limit!"