The reason why they initially chose the 757 was (i) it had transcontinental range (ii) It was a boeing, with whom they were doing considerable 737 business at the time, (iii) there were large amounts of ex-Eastern Airlines models that could be acquired, cheaply and quickly, and (iii) it has a common cockpit with the 767, a type they already operated.
US Airways didn't begin operating any Airbus A320 family aircraft until 1998, and the earlier models of the A321 did not have anything close to transcontinental range, which is what US Airways needed. Previous to this all transcontinental runs were on 737-300's (save a few that were on 762's), which were severely streched to go PHL-SFO/LAX without weight ristrictions. Often, they did not make it and had to make an unscheduled fuel stop. Eastern Air Lines (who was the largest operator of 757's at the time) had recently gone out of business, and due to a global economic recession, these aircraft were having trouble being placed with other airlines. To cut a long story short, US Airways got their first 15 757's for a song from Eastern's creditors (plus they were available immediately).
In 1999/2000 Airbus launched an upgraded A321, which had enough range to fly transcontinental. US Airways, already having A319's, A320's, and A330's, was able to convert orders that they already had into these A321's, for the same price. They chose this to augment their transcontinental fleet, rather than deal with Boeing--whom they had just settled a lawsuit with over US427.
BTW, the 757 will probably be one of the next aircraft types (after the F-100, MD-80, and 737-200) to be retired; Due to the fact that they only have 34 and the ex-eastern models are approaching 25 years old.