I have a couple of scenarios I'd like to share that have both happened:

Scenario 1 happened in MDW in May. I can't confirm what happened, but I believe weather was involved. It appeared that nearly EVERY WN flight was changed (meaning, the aircraft was swapped at MDW...so if you were on a 1 stop same flight number CMH-MDW-MSP for example, you'd have to get off and change in MDW). This caused some big delays and they blamed weather. However, the point of the story is they swapped things around so that the delays would impact the fewest number of people for the shortest time. Obviously loads were weighed as to which planes went where. I know I was concerned because my family was on MDW-FLL and that aircraft turned right back around to MDW but the flight was just delayed and not canceled.

As someone said, loads and other impacts later in the day also effect the decision. Back in 2008 I believe, I was on FLL-TPA (or MCO...I used to connect at both and my memory is a bit foggy). The plane had come in from TPA as the first flight of the day. THERE WAS ONE PASSENGER ON THE INBOUND! They could have easily canceled the flight but didn't.

So I will agree with others. Loads and convenience are part of what determines the completion of the flight. That being said, I'm sure ALL airlines strive for a 100% completion and I'm sure there was no bad intentions in canceling the OP's flight.