Canada supplies 40% of US steel.
Canada supplies 80% of US potash.
Canada supplies 40% of US crude oil and 20% of US natural gas.
Canada supplies 30% of US hydroelectricity.
Canada supplies 24% of US lithium.
Canada supplies all this at discount rates in exchange for market access. A tariff war means that ALL US industries that rely on these materials will pay more for them. The US cannot possibly resource them locally in sufficient quantities to meet demand, the infrastructure doesn't exist to produce them.
Secondly, declaring a trade war on your greatest ally is a clear message to the rest of the world to NOT trust the USA. Who wants to do business with a country that does not keep its word and tears up trade agreements like the USMCA (which His Royal Orangeness HIMSELF signed in his first term) for no good reason?