I've mentioned the border thing here many times...And while technically, importing a car older than 25 years into the US is 'easy' it's still a pain in the butt, especially if it's not something you do regularly. As Canadians, we're usually pretty familiar with at least crossing the border, and maybe bringing stuff back across. That's a foreign concept to many Americans, many of whom have never crossed into Canada, let alone imported anything from there. For the majority of American collectors, it would either need to be real deal, or an extremely rare car, to justify even considering importing. That's just how it is.
Now, importing a car <25 years old into the US is actually a royal PITA. I am going through it now for my junkers and it's just unnecessarily complicated. Much easier to import from the US into Canada.