It’s not *because* it’s so far north, but it is a transition that typically occurs as tropical systems move to more northern latitudes (although it doesn’t necessarily have to wait until it’s as far north as Canada). I’m no expert, and hopefully someone else will provide a better and more technical answer, but basically a tropical system undergoes a transition whereby it loses its tropical characteristics / structure and becomes more like a typical mid-latitude cyclone. Sometimes it combines or gets caught up with other existing frontal boundaries or mid-latitude storms. There are some specific criteria, like it no longer has a “warm core” and it is more baroclinically driven, i.e., driven by temperature contrasts and frontal boundaries. If you look at it on satellite, it transitions from a tight circular spiral to the familiar “comma” shape of typical mid-latitude storm systems that you see traverse the continental US anytime of year, containing a warm sector and a cold front. There are specific differences between post-tropical, extratropical and subtropical. You can learn more on the NHC site.
This classification issue caused some interesting issues with Sandy back in 2012. At some point it was no longer a hurricane or tropical storm, but still had hurricane strength winds and storm surge. Back then, once it was no longer a tropical system, IIRC warning responsibility shifted from NHC to local NWS offices. There was a lot of confusion and mixed messaging as a result - people thought they were safe because it was no longer a hurricane, and yet got slammed by wind and especially surge. The general public, of course, does not care about these technical distinctions between tropical, subtropical, extratropical, etc. Changes were made going forward, so I believe now NHC continues to be responsible for warnings on the storm even when it is no longer “officially” a tropical system. Also with Sandy I heard there were insurance issues, whether or not it was “officially” a hurricane affected whether policies covered the damage. I can’t remember the details, but if you’re interested you can Google it. A good source to read about the warning issue, if you’re interested at the “inside baseball” level, is the NWS Service Assessment on Hurricane Sandy, which is publicly available.
