Canada imports more from USA than it exports to USA. A trade war that decreases exports would hurt USA more than Canada.

The largest exports from USA to Canada are cars and trucks. (Finished products) The largest exports from Canada to USA are minerals, fuels and pulp. (Raw material)

This puts USA at a disadvantage in a tariff game, because it's a lot easier for Canada to replace American cars with Asian or European cars, than it is for USA to replace the variety of ressources that Canada provides

It's also a lot easier for Canada to find other customers for raw materials than it is to sell American cars on other continents.