If they had decent merchandise managers, they would have balanced the demand with the supply.
But seems Disney is "make as much as they can" then "sell it everywhere later" with no sense of exclusivity.
This is NOT as easy as one would think. Especially, when it comes to special event, limited merchandise. The inaugural cruises or special cruises like Panama Canal have a higher than normal volume of people who are resellers. People who attended the 90's era Disneyana events, or pin events, know this type. It happened before Ebay, and Ebay has made it a thousand times worse. Before DCL started making larger volumes of inaugural merchandise, the resellers would get themselves in the first boarding group, immediately go line up at the store, and within an hour of the store opening, every single piece of special merchandise would be sold out. So any normal guest, who were unaware of this type of chaos would visit the store and wonder why nothing was available.
These sellers upset the balance in unpredictable ways, because you don't know what level of availability causes them to "give up" and not buy as much as they can, and only buy for themselves and a few friends/regular customers. Demand for an item may be 500 pieces, but it takes 2000 pieces as being available for the resellers to modify their behavior. So if Disney made 1000 pieces - instant sell out, but 2000 means only 500 sells.
There have been various ways Disney has tried to combat this. We went on the first trans-Atlantic cruise, and they ran some of that merchandise like the RSP system they use for the yearly pin event. Each room fills out a sheet with what they wanted, and then mid-cruise, you are told which items you "won" and were eligible to buy. But that takes a fair amount of work. My parents went on a Panama Canal cruise where many things sold out, and they were given a form to fill out with shirt sizes, and when the ship reached one of the later ports, the shirts had arrived. But being made last minute, you can guess the quality... And this only was for the shirts.
Personally, I think many things should be on a "pre-order" system, and use that to gauge the final order. But I don't know how well that works with Disney's "lead time." How early to do you need to have the merchandise announced, how early to shut down pre-order sales to have enough time for the merchandise to make the ship. These inaugural sailings should book early enough that people will at least know they are going or not, but some people want these things to be a "surprise."