There are several factors.
First, SSR and OKW are the only DVC resorts that don't really have much of a "location advantage." Most of the others are close to one or more theme parks, allowing you to avoid a bus to get back and forth. AKV doesn't avoid buses, but it does have the animals on site, which many people enjoy.
Second, SSR is one of the resorts using the "2nd generation" resort design that DVC adopted. (The other 2nd Gen resorts are VWL, BCV, and BWV.) This design has too little common area space for the sleeping capacity of the unit, balconies that are barely functional, kitchen appliances that are not full sized, and with other corners cut here and there. In contrast, OKW units are among the largest in the system, with lovely verandas overlooking lush landscaping.
Third, there's a certain snobbishness among TUGgers that if something is easy to exchange into, it must not be worth the exchange. SSR is a huge resort, and the last one to book internally, so it has long been the one deposited most often, though not quite as imbalanced as it is these days. Back when OKW was easier to get into, no one wanted that one either, but now that it's a harder exchange TUGgers are eager to take it. ;-)
I do think SSR is on the upswing. As Disney Springs continues its development, it will once again provide a nice draw for SSR---and I think we are already seeing that with BOATHOUSE, Morimoto Asia, the soon-to-open STK, etc. That draw really faded when Pleasure Island was first gutted, and later closed entirely. Furthermore, when the resort first opened the landscaping left a lot to be desired, as it was a bit of a moonscape. However, over the years, the landscaping has really started to come into its own, making the environment much more pleasant. Finally, the new amenities by the Paddock have improved that area, and the new Preferred/General booking categories may be good news for exchangers, though the latter remains to be seen.