Supply and Demand.
The timeshare development model is based on getting all the profit up front at sale time. (Except Marriott DC point, which demand a cut every time they are traded.) What happens after doesn't matter to the developers, except for how it would affect the ongoing sales process. (That's why branded tend to do better that non-branded. They always have an ongoing sales process, somewhere. Having prospects trade into another branded property that is junk hurts the image. . . )
Otherwise the developers DON'T CARE what happens to a finished property. They only care about getting those units sold, whether or not to people who are suitable for the timeshare concept or not. Sell that unit, SELL THAT UNIT!
The long term result is a lot of timeshares sold, and many more our there to people who aren't a match to the product.
There are more units out there than there are people who want to use them for their intended use. In any product/market, when that happens, the value goes to $0 (or less). (A generic comment. Some systems/properties managed to "beat the system" with a stable ownership clientèle. They are few and far between, however. There are other niches, if you look hard for them. . .)
But on goes the building of new units. . . .