There is indeed a case for AC
to operate the B757 but they have decided to go with the high gross weight A321-200 which does an excellent job for AC
but lacks the range capabilities of the 757. The A321 has mostly replaced the old non-ER 767-200s that did yeoman's work for AC
for over 20 years on domestic trunk routes (there are not many left). Therefore, the extra range and payload capacity of the 757 would be largely wasted in such a role and the A321 is a bit more fuel efficient.
Engine commonality may also share a part in the decision - as AC
has not operated any RB
.211 aircraft for almost 10 years and has never used the PW
2000 series. The A319/320/321 all use CFM56 engines but obviously not all the same model. The A319 uses the CFM56-5A with about 23,000 pounds of thrust while the A321 uses the CFM56-5B2 with a healthy 33,000 lbs of thrust. This is almost as much thrust as the initial RR
engines offered on the 757-200. The power on takeoff is actually quite impressive.
The 757 would make more sense if AC
wanted to open up more "thin" transatlantic routes to Europe in the way that CO
has...but that must have obviously not been a priority to serve markets with anything less than a 762ER.