So it seems that a lot of cruise ship itineraries are on a 14 day schedule - 9/5 or 5/5/4, with an occasional 21 day schedule - 8/8/5 or 12/9. Is there a particular reason for this other than operator convenience?
Actually the average length of a cruise ship journey is 7.2 days, so you may find that's just confirmation bias that you're seeing.
Regardless, like any other business model, it's likely that it's based on supply and demand, that those lengths are the most popular with customers who want two weeks off work or have to fly to the start/end and want a gap in between. So from surveys, responses, sales and feedback, those will just be the lengths that they've settled on.