There are two issues.
The first, and most obvious one, is that RCI has very well-developed rental channels, both to other RCI members and non-owners alike. This is good news for Disney, as they have a lot of inventory backing "Reservation Status" points that they need to rent out and turn into cash. RCI's rental market is ripe for this, because it consists of people who already know why condos are better for them than hotel rooms, and are willing to pay for the extra space and comfort.
The second reason is the one that The Powers That Be give: RCI's larger inventory base has allowed them to satisfy more exchange requests more quickly than II did. Whether you believe that or not depends on your level of paranoia when it comes to TPTB. But, i suspect it is likely to be true. As Dean points out, a lot of those "great" resorts in II get very little inventory---particularly in season. What's more, the Marriotts and Starwoods all have internal preference periods, where other owners in those systems have a window of exclusive access to newly deposited weeks. Finally, since DVC left, Marriott has gone to their Club product, and significantly changed the way they deposit, to the detriment of II exchangers---this is something that the II/Marriott folks on TUG are very very unhappy about.
Ultimately, for an informed owner, it doesn't matter with which exchange DVC affiliates---on a cash-rental basis, almost *none* of them are good deals. You would almost certainly be better off renting out points, and using the proceeds to secure the non-Disney lodging you desire.
The BVTC thing would be interesting, but the question is: what's in it for Disney? How does having it help Disney sell more timeshares? My guess is that the Guides have learned how to cherry-pick destinations and resorts in the RCI "wish book" to make it look like an attractive alternative, so the marginal value of "better" alternatives on the sales floor is modest. But, spinning up a "real" exchange system would cost real money, and Mickey would want a return on that investment---*and* would need to find a way to replace RCI's rental channels.
I think switching back to II is more likely than a BVTC spin-up from a business-case perspective, and that would happen only if II gets serious about its own rental channels.
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