Could be many reasons, but some possible ones that I can think of:

Bar Harbor could've come in and said "we've got too many other ships in port that day, you lose"

Mechanical issues/fuel issues saying they need to go slower, hence the sea day.

Recalculation of tides meaning an issue with original port times and can't change times due to other issues so just lose the port.

When they can they do try and substitute ports (like happened with Portland for Bar Harbor) so it's possible there wasn't another viable port for Shelbourn, N.S.



I've had itinerary changes in advance and others mid-cruise. If the changes will really affect your enjoyment of the cruise, then I would recommend cancelling. But if you can look at it as an adventure and an opportunity to do something else, you can still really enjoy yourself.