Why is meter spelled differently in Canada?

Answer 1

The "why" is probably for historical reasons. In general, several words are spelled differently (compared to the U.S.), not only in Canada, but in Great Britain as well; this is usually known as the "British spelling". Some changes are meter --> metre, liter --> litre, and other similar words. Another large group of words whose spelling changes is color --> colour, labor --> labour, flavor --> flavour, etc. (I put the American U.S. spelling first, the British spelling second, in these examples.)

Answer 2

The difference in spelling (metre vs. meter) is due to British English and American English spelling conventions. In Canada, British English spelling is generally followed, which is why "metre" is used in Canadian English.