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.
Delta does not want to be a holding station for thousands of cruise passengers. Imagine a warehouse full of bags for each ship and the ships have gone from 1-2 thousand to well above 3-4 thousand passengers. A lot of cruise customers take two bags each and some I know take 3-4 for bags each!