Check-in times can be considerably longer so they are requiring people to not check in any more than 2 hours before departure? Now that makes no sense, but par for the course for DL.
The cruise lines drop off bus loads of passengers all around the same time (9am +- an hour). Some of those passengers don't have flights until later that afternoon, so they don't want those people clogging up the lines for those who have flights in the morning. Otherwise a regular passenger on a 10am flight could find himself behind several bus loads of passengers who don't fly until after noon, making it difficult for him to get through the line on time and make his flight.