I checked the flight in Expert Flyer. It is fully sold out. J0 Y0. While it seems odd, American is not offering seats for sale even with a lightly-seat-selected cabin.

SJD to DFW is J1 Y4 so there are only 5 seats lefton the plane. Perhaps there is a conference ending on that day? Cruise line booked a plane full (and cruises don't typically reassign seats in advance).