I remain convinced that at least
some of the decision is based on the Unit the points belong to. While
DVC can change UY's to match direct purchase/add-on requests, they cannot change the Unit the points belong to, and can't combine points from more than one unit to create a larger contract (25 points from Unit ABC, added to 25 from Unit DEF to create a 50-point direct contract for example).
If I call and say I want to buy 200 points at PVB, and they only have 150 and those belong to Unit ABC, then they need to look for 50 points, also from Unit ABC, to grab through ROFR to get me my 200. They stick me on a waitlist until they find them. I'm sure decisions made to ROFR, just to maintain some inventory, are also driven, to some extent, by Unit as well as they probably want to have a bit of distribution in what they have on the shelf.
Keep in mind, most resorts are comprised of at least several dozen units, with points spread over tens of thousands of contracts. DVC might have 25,000 points (an arbitrary number) on the shelf at PVB (my guess is the goal is to have some but not too many points sitting around), but they could be spread over hundreds of contracts and getting a larger contract combined for a direct or add-on sale still might entail some cobbling on their part.