The airline can swap their slots around from flight to flight however they want.

There are many factors which can prevent an equipment or crew swap to reduce a flight's delay. No way to know what it might have been in this situation. They do it often, however, so it was likely considered and rejected for a good reason.

I was once pulled off a flight just as the jetbridge was being pulled in order to operate a different flight that was delayed. The crew initially scheduled for that flight would have timed out due to the long delay but they were legal to fly my flight so we were swapped. They traded taking a delay on my original flight for not cancelling the other flight.