They are using Halfmoon Cay on the other 5-night cruises which leave on Saturdays. Only the Monday departures are using Coco Cay. It is a usage issue. Halfmoon has other ships there on Thursdays, but Coco doesn't.
Now here is another thing to consider - if Thursday was a routine 'off-day' at Coco Cay where the live-in crew could relax, or perhaps do maintenance and upkeep, when will that be done now?