Because the US was there to keep the USSR military from coming across the Bering Strait
Without the US, Canada would have been a sitting duck.
That is how I see it. I shrug my shoulders at seeing any other possible outcome.
Like I said, if there was no America and Canada had America’s resources, it would probably have a much stronger military. But because America was there, Canada had no need to build a multi-billion dollar military. If you argue on that, you might have a point.
Other than that, I don't see what was keeping the massive USSR military out of North America other than America.