@renee555,
I've never run into this situation but I guess it would depend somewhat on whether everyone was called at the last minute to be advised the wedding was cancelled, or the bride and groom just didn't show.
My guess is that the more likely scenario was be that people are called at the last minute and told but some people aren't reached and they would have made the effort for nothing, been disappointed and saddened and gone home.
Or the other likely situation would be that the groom or bride would show up and the other not arrive and everyone waits and waits and they finally give up, or the one person advises everyone that the wedding is off and they apologize.
Whatever the situation it would be very uncomfortable for everyone of course but better in the long run than marrying if not right and ending in divorce perhaps with children involved.