AC and YYZ-PHX have always puzzled me. I never see cheap fares but AC doesnt seem eager to add many seats. (I know AC went from 1* A321 to 2 * 319).
I know there is not much business in Phoenix but there are a lot of 5* resorts, a lot of Big time confrences (in fact, a couple trips ago I saw an AC conference/meeting at my resort in Scottsdale), and there are a lot of wealthy PAX heading down for golf / sun etc.

US Airways also pulled out of YYZ nonstop so I just to get it I guess. Though, if AC and US were closer partners, I'm sure we'd see more nonstop service from each of them