A contributing factor may be the number of American car companies with factories hereβI mean, there are a few European/Japanese/whatever auto brands that do some manufacturing here too, but not as much. It's an industry that seems to have a political voice that's larger than its contribution to the national economy (or at least, that's the impression one gets from the news here in Ontario). Lobbying to tweak the rules to make certification easier for American vehicles than others seems on-brand.
If that is part of the reason, Trump may have torpedoed it, but it'll take years for the mess to untangle itself even so.