[Targan]

Remember, it wasn't a general full-scale nuclear exchange like we've all grown up fearing. There was no MAD scenario. It was a slow escalation, with all sides involved pushing things just far enough that they didn't kick off a full-scale, all-out ICBM exchange. Over the couple of years that the tit-for-tat nukings occurred there would have been countless thousands of meetings and discussions and little events and evaluations that would be too numerous for even the most devoted alternate history buffs to flesh out. Unless you're re-writing the timeline of the war and it's nuclear exchanges (and many here have or are in the process of doing just that) there's no point second guessing why some targets were deliberately left off either side's hit lists. Launch failures, targeting problems, detonation failures and the like are a whole other issue though.

It may be that the Soviets nuked major population centres in Canada but not in the US because they judged that nuking major population centres in the US would be likely to kick off a full-scale MAD scenario. There's no risk of that with Canada. They don't have nukes.