Kind of proved the point of America being an un-reliable partner which is what I inferred from Canadian PM's speech & his call for middle economies to connect with each other and strengthen together to have more leverage overall.
The previous Canada-US relationship is gone. Months ago I wrote on HN that purely by virtue of having to weather this storm, the nature of Canada-US relations will be irrevocably and fundamentally altered. Even if Trump and his cronies were jailed tomorrow, it's too late. The rest of the world understands that Trump is just a symptom of the disease affecting America and it's going to get worse, not better.
In addition to that, since we're on the car angle, Chinese EVs are basically just privacy nightmares. I mean, all cars are at this point, but that's why we definitely don't want Chinese ones coming across the Canadian border and ending up all over the place.
In the end there are in fact legitimate national security concerns that the tariffs address and Canada risks weakening those. So, that is the actual answer to why.