The reason is because Tesla wants to protect their new car market in Canada. They don't want used US vehicles to compete with their new car sales in Canada. There's also an issue in the US with Tesla not having dealerships. It limits their ability to sell used cars privately.I think the major differences are KM vs miles in the dash (easy), red seat belt release buttons, and always-on DRLs.
From what I recall in other threads on this issue, there is an approved list of cars that can be imported into Canada. Teslas are not on the list. Canada is open and willing to put Tesla on the list, but for some reason, Tesla is not (at this point) willing to work with the Canada government to put Tesla on the list. I don't think it has to do with specific hardware requirements, as most US cars already conform (or are trivial to change with software). And it's not about milking Canadian customers. I think it's just Tesla doesn't have the desire and/or the resources required to give Canada what they want to add Tesla to the list. I don't think it's a conspiracy.
I understand the issues for Tesla. But it's very frustrating that they are doing nothing to help me in my situation.