The market is playing itself out here. It is the peak travel season for such trips and you are expecting to get a really cheap fare? Get real and face facts: airlines are in the business of making money and hopefully a profit, not subsidizing your travel. Given the demand for seats on this and other routes, carriers are able to charge a premium for whatever seats are left in their inventory. Several months ago, you could have found cheaper seats, but these are now all sold out. In fact, AC has had several transborder fare sales through the spring and you could have picked seats up for 30% or more less than those you quote. Any seats now remaining are really aimed at the short notice, business traveller. This is how the marketplace has sorted itself out.

As noted, you can try Harmony, or Alaska, or now Qantas on this route. All have likely sold out their cheapest seats...again because this is the peak of vacation travel. Sebring has posted a pretty complete explanation, but some people just don't want to accept the realities of air travel today after the last decade of give-away pricing that drove just about every NAmerican carrier into bankruptcy.