The US Airways side took delivery of the first A321 in January 2001 - pre 9/11
https://aviationweek.com/us-airways-...st-airbus-a321

First flight of the 737-900 (non-ER) was August 2000.
https://aviationweek.com/boeing-737-...s-first-flight

The biggest challenge of aircraft of that era was the lack of transcon range. Legacy AA was focused on fleet simplicity. It acquired TWA and then 9/11 happened and all the airlines were in survival mode with few order purchases. Legacy US airways needed Airbus financing.

The MD-80s on the legacy AA side didn't have transcon range. The 739 did not have transcon range, but the 738 did, and was a better fit from a size and role basis.

US used their A321s on short trunk routes - like LGA-CLT. The early A321s didn't have transcon range, but improvements and later versions certainly did. US used the A319 for transcons during this time.

When pre-merger AA made their pre-bankruptcy split order between Boeing and Airbus to replace the MD-80s - they decided on the 738 and an A321 split - with each aircraft being better suited for its mission. AA picked the 321 over the 739, but also picked the 738 over the a320.

Historically - in terms of why Continental and Delta ordered 739s - part of this was a legacy gentleman's agreement to only buy Boeing to get better pricing. The early 739s had limited range. Range improved with the 739ERs,
https://www.spokesman.com/stories/19...ine-considers/