I assume that you checked that the underlying AA flight had also been cancelled. Sometimes, the codeshare gets removed as a result of a timetable change. In that case, it is usually best to wait a few weeks (if you itinerary permits) for the codeshare Gods to get to work on the revised schedule and for the new BA number to appear. If the underlying AA flight has been cancelled as well, and there are no other options on the day, then I should say that you have been rather unlucky. It is not typical.

These days, since the changes in Avios earning, there is not much point holding out for an BA codeshare if the AA prime is available (unless you are doing the BAH double points thing).