The leases expire early next year, they're more or less incompatible with the existing AA 767 fleet (different engines, different maintenance requirements), and were no longer needed with the winter schedules that went into effect November 1.
We could probably keep flying them, but there's so much excess capacity in the system right now that it doesn't make sense to keep a dwindling sub-fleet in the air, especially since operating cost increase as the subfleet shrinks.