Any change to the flight operations within a couple of days of the flight is more of a nuisance to the airline in rearranged crewing, maintenance, etc than it tends to save. So generally this only happens when circumstances force it. For all that a crew is saved on the larger aircraft, another crew (usually the crew have different qualifications so are not interchangeable) must be found for the smaller aircraft. And then the aircraft are out of position for their next assignments.

Then there are all the negative commercial downsides. One cancellation probably undoes 100 positive advertisements in passengers minds.

Also airlines do not have smaller aircraft, which cost £ millions in their own right, just sat around waiting in case a flight has a low load factor. The whole fleet is normally tightly programmed.

Most likely reasons are the good old favourites, mechanical problem or late arival of the incoming aircraft.