For literally joining the EU: geography, Hungary and the optics of them leaving out places like Kosovo and Bosnia while we go ahead.

For getting every part of EU membership except the technical status, there's much fewer. The only substantial thing I've heard is that they're afraid of Russia and there's a limit to how much we can help with that, while on the other side they're tempted to just avoid confrontation with the US, or maybe even (wrongly) think they can keep the alliance going.

I don't know what OP was thinking of, though.

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.