Surely it must be about the 787 planes themselves. I can think of several good reasons to fly to Calgary in winter, but very few for summer...

As for YYZ... if a British airport got as much snow and ice as Eastern Canada it would be unable to operate for three months. But since the major Canadian airports actually invest in being prepared for winter, the impact is manageable.

But yes, it does make sense to backtrack to YVR and fly over the pole, rather than cross-country then TATL.