Circumstances change and can change quickly, but there was an interview with Cuscuna recently which seemed to indicate that they planned to continue (was more positive than i'd imagined).
I think as they do small-ish print runs of say 500-1000 at a time (rather than printing the whole 7500 copies of a set all at once and storing them), i think depending on how the set has sold maybe sometimes they opt to let it go OOP voluntarily rather than printing another 500-1000 or whatever. So if it's been sitting on 'this item is backordered' status for a while, maybe they look at it and go, "guys, we ain't gonna sell another 1000 of these, rather than printing 1000 we've got a few returns lying around that we can tidy up and cobble together and put it on 'last chance', and then that's it".
Or maybe the licence runs out. I guess it's case by case.
This is pure speculation on my part.