Because the A310-300 exists. It's a 757-300ER.
Little Problem, if you are expanding you won´t be that dumb to order the the A310-meh, but rather order it´s bigger, more beautiful and more fuel efficient per seat brother the A300-600R, although the 753 may have 1000 miles less range but it is 20% (without WL) more fuel efficient than a A310 plus this 1000 miles gap can be plugged any time with the Kitchen Gun of widebodies, the 767-300ER.
And when we speak of the 757 than I highly recommend to order the -200M instead of the -200 because deliveries happens every 2 weeks instead of three which in the game year of 40 weeks means 20 757-200M against 13 1/3 757-200 and tbh range is just overestimated, it´s efficiency that is all above else.