DVC is a timeshare. That means rooms have been purchased by the members. DVC does not overbook like a hotel. Only rooms that have been purchased are available for members. DVC operates at 95%-98% occupancy. When a member trades out for a cruise, RCI, or any other outside option, they effective rent out their time (their room) to Disney, who then sells it on the open market. Once Disney holds the room, it doesn't belong to DVC members anymore. That's why there are cash rooms available, but no points rooms.
The entire points schedule is designed to make sure there is no "low" season. When there is typically less demand, prices (points costs) are lower. When demand is higher, so are prices. Don't expect there to be a low season, and, if you need a last-minute reservation, you'll need to be flexible.