The flight was scheduled as a 757. American equipment subbed it to a Airbus 321 with the same seating capacity. They would have had to block off 40 seats to get to clt. Instead they put a light load of fuel on and refueled at SJU.
Airlines asked Boeing to reopen the 757 production line and consider newer engines. Boeing instead has been pushing the 737-900. It suffers from the same performance issues as the Airbus. Both the bus and the 737-900 are considerably cheaper on a per seat basis to operate. Delta and other airlines are overhauling 757's to extend the service life. Low cycle 757's are commanding very strong resale values. A reengined 757 could probably due STT to LAX or even SEA nonstop.