I asked my local Canadian dealer why the 2019 base Model S isn't available in Canada, because it was available in previous years. His answer surprised me. He said it's because backup cameras are required and the Model S has no backup camera (no entertainment system = no screen).
I've done a little digging and a backup camera law also came into effect in the US at the same time (the Canadians pretty much copy the US to match a lot of the regulations across the border).
So why is the Model S still available in the US and not in Canada?
I believe it's because the federal government in Canada classifies the SlingShot as a "three wheel vehicle (TWV)" and a backup camera is now mandatory on all vehicles built after May 1 2018 weighing under 10,000 pounds.
In the US I believe the federal government still classifies the SlingShot as a motorcycle, so it escapes the law.
It's pretty frustrating to be denied the choice in Canada.