I think AA is very "nervous" about the hassle and expense of pushing an aircraft back and having it come back to the gate due to the 3 hour rule.

It has to be a nightmare for them to deal with finding a parking place and then a ground crew to deal with the plane, passengers, luggage and a crew that will likely time out.

Cancelling early in the day means it is not even an issue if there is even a chance this could occur.

Other airlines seems to take the risk about pushing back and having to return a lot more frequently than AA.