Prices go up and down for 3 general reasons 1. Scarcity 2. Perceived value 3. Market control (a monopoly or the economy)
Remember that TimeShares are a long-term commitment with costs (MFs) that go up every year. The first few years of any point system, most owners are new and happy owners; the longer that point system lasts, the more people who will be living in a very different economic system from what they had when they first signed up, the more people who will have decided the TS isn't for them, the more people whose health or family situation makes it harder to make use of the TS, and the more people who want the most popular/hardest to get into resorts. So the longer the TS has been around, the higher the percentage of owners wanting to sell.
Plus Timeshares depend on their sales department much more heavily than most other purchases. Nobody
needs to own DVC, and most people who buy DVC, buy because a sales presentation convinced them it was a good deal. DVC salespeople are a not so high pressure as your average TS salesperson, but they still count heavily on appealing to the buyer's emotions. Very, very few people buy DVC for purely rational reasons, because the purely rational approach would be to get bigger, nicer accommodations offsite, and then trade into DVC when possible. You buy DVC because you enjoy the magic and want to be part of it, and an amazing percentage of people buy DVC without doing much research at all.
All those people who buy into DVC without doing research are not part of the market for resales, so there's only a very small overlap. The usual market for Timeshare Resales, which is to say, people who already own and understand Timeshares, is a totally different market, and most of them think it makes more sense to trade into DVC anyhow. People buying on the resale market also do more research into which resort trades best, which MFs are higher, are more interested in minimizing costs, etc. than people buying direct from Disney.
Disney is selling DVC to a different market than people who are trying to resell their points are. There's also the fact that Disney can wait out tough economic times; many owners who are putting their points on the market
need to sell those points, meaning they're much more willing to negotiate and much more likely to drop their price. Two different types of sellers, selling in two different markets, means two different sets of prices that are not very tightly connected.