Actually 220-270 pax is almost exactly the capacity of a 757-300. When they are configured with premium cabins, capacity runs about 210-235 pax. If crammed full of economy pax, Boeing claimed one could carry 285 pax.

The 737-900 replaced the 757-200. The 797 will replace the 757-300 and fill the gap between the 737-900 and the 787.

Neither 757 variant has been manufactured since 2004. The newest will be 20+ years old by the time the 797 is available. Besides meeting needs for more fuel efficient aircraft, a lot of the 757's will be retiring.

Loading & unloading a full 757-300 is a bear. The 797 will be a twin-aisle aircraft and likely will re-use a lot of components from the 787 design, just as the 707/727/737/757 were evolutions of the original 707 design.