It's not as dramatic as you state. Westjet buys lots of 737ng's, and the 767 was popular with Canadian/Air Canada. The only thing is, Boeing lost the battle with Airbus when it comes to Air Canada; apart from their 767s and a few 744s, they are very Airbus oriented.
Some stuff might be factors;
1- Canada being culturally and economically more oriented on Europe (as member of the British Commonworld and with their French-speaking states) than the US.
2- Airbus probably offered planes which were fitting better in Air Canada's fleet and/or offering them cheap to get a hold in the Canadian market (and finally having 330/340 widebodies flying in the America's).
nobody has ever died from hard work, but why take the risk?