There are all sorts of reasons/scenarios for cancellations, many of which are too complex to start explaining to pax, who would only respond with, "why our flight"?
If an a/c goes sick for destination A, which has only 1 rotation per day and is heavily booked, it may be that an a/c planned for a rotation to destination B, which is served with 4 daily rotations - with lower loads, will be substituted for the 'A' run. The 'B' run pax are put on the next B rotation. An airline's ops dept is continously juggling with a myriad of problems and will strive to find the best possible solution for all parties.