I'm not sure where @SWDigits comment went, but he indicated there were lots of storms in the DC area yesterday. This cancelation does appear to be a result of that as the plane that was scheduled to fly ATL-AUS instead went ATL-DCA this morning.
The "didn't sell enough tickets" reason makes sense in a traditional hub and spoke situation, but makes no sense with Southwest as the plane can't just not go without affecting the 5+ stops it is scheduled to make the rest of the day. When bad weather or mechanical issues screw things up there is a ripple affect and unfortunately you were impacted by one of those ripples.
--TheMiddleSeat
