You’d have to get the entire union to agree. Including Hungary. Canada would have to change a bunch of stuff around when it comes to movement, currency, etc. The US would likely fight the move by putting its own NATO membership on the table. Assuming there haven’t been any Article 5 violations, then the EU would consider that a very bad deal. The list goes on.

Now, Canada and the EU increasing trade and allowing freedom of movement is not entirely impossible. I think new alliances are inevitable. I just don’t see Canada in the EU as a likely scenario.