Basically for most
travel insurance to kick in under trip interruption for cruises your flights need to be booked 24 hours prior to all aboard (including enough time to arrive at port)
The chances of zero flights being available for 24 hours is extremely rare but what is more likely is a flight that say gets you in at 5PM the day of sailing and you still missing your cruise. 6 PM was not the best example but many ppl take flights that leave after work and arrive late the night before in which case travel insurance is not going to help unless you have cancel for any reason coverage