I believe DCA requires specially trained pilots given the limited approach/ take off rules and added security protocols flying into the airport which is another limiting factor besides the aircraft difference types.

another factor is the weather and crew availability and aircraft coming in

another factor is rebooking options. You can look at PAX profile and see how many on this plane are local metro Seattle residents vs connections/ people heading home. If it’s 80% SEA/ PDX area locals, it’s likely easier to rebook the non locals on other flights to IAD and BWI and connect through PDX that day and the next 2 days.

I don’t know how easy it is for them to do splittebookings using AA through DFW, ORD, BNA, And other central/ east airports that AA has DCA direct flights.