Not sure that the Westjet alliance with KLM and Air France is the only reason.
This year, Westjet started their own vacation division, just as Air Canada has Air Canada Vacations as a subsidiary. Westjet offers all-inclusive packages to Mexico and Vegas this season, but I’m sure they’d like to expand those offerings. Any expansion would require more of Westjet’s aircraft to be available which is why I suspect they dropped their contract with Air Transat.
Personally, I love Westjet and they have become my domestic air carrier of choice. Westjet is a western Canadian airline based in Calgary. I always found it bizarre that people in central Canada and the Maritimes could fly Westjet to Cuba, but we here in Vancouver could not. Vancouver vacationers were only able to fly Air Canada (via Toronto or Montreal), Sunwing, Skyservice or Air Transat.
I’ve never had the pleasure of experiencing Canjet and only got to try Sunwing this year as they are also new to the Vancouver market. We had no issues with Sunwing and found them better than Skyservice (who offer absurd flight times to Cuba from Vancouver).
I’m keeping my fingers crossed that Westjet Vacations will expand their service from Vancouver to include Cuba for next season.