I expect that the thinking goes along the line of since transplant surgery only occurs in specialist centres, and where the recipients are already very ill and will probably require intensive care post-surgery, whereas potential donors could be in any major hospital site then both the suitability of the organs and practicalities of getting a donor organ to the recipient in time for use.
Presumably the idea is that since transplant patients often have a long history of illness then when the need for a donor organ is needed then (assuming the science works) one sourced from pigs could be arranged without the need to await the death of a suitable human donor, even where the practicalities of acquiring and transporting the organ are do-able.
In essence some pigs will be bred as organ donors, possible for specified patients, and then slaughtered at the point (or nearby) when the organ is needed.
It sounds a bit ghoulish I suppose, but probably no more so than slaughtering pigs to a schedule needed to supply supermarkets with bacon - of course there are different views on this whether using animals as a resource (such as for food) at all.