This is most likely because AC/CP is using their smaller aircrafts for transborder flights and for new destinations/frequency that they are implementing. They already have about 13 flights per day every hour so there's no point adding an extra flight at the end of the day which they cannot fill to capacity and to "waste" an aircraft overnighting in YVR.

With the strong competition from Royal and C3000, most leisure traveller will be opting to fly the evening charter flights unless AC/CP has a seat sale.

Jeff: If you fly to the US exclusively, why not consider switching to UA and become a 1K where you could upgrade since AC is no longer an option for you?