They have to have a departure time which allows for passenger arrivals at the airports. Despite our advice, many people still fly in on the day of the cruise. Sales would likely plummet if the cruise line said "arrive a day early so you can check in and we can leave the following morning". It won't sell with the typical Alaska cruise audience. The cruise lines have already pulled several ships from the Alaska market. Requiring people to show up a day early would only send it further down.