9.) The entire ship reeks of cat piss. It's completely unbearable to the NPCs. The PCs get used to it.
10.) Don't bother to give the PCs a reason not to sell their ship. However, if they ever actually attempt to sell it, have them get ripped off by a con artist, and plan an adventure around getting their ship back.
11.) The PCs are Couriers and the ship itself is their delivery package.
12.) The ship is a rental from Abadarcorp and the deposit is larger than what anyone is willing to pay for.
13.) One of the lead crew members of the ship is a Mystic or other religious character. They believe this is ship is blessed and is a literal 'vessel' for their deity.