As already posted, MS only makes rooms available for the Members' cash discount if they think that all of the rooms in the points inventory will not be reserved by members with points.
For your dates, they either think all the rooms will eventually be reserved by members using their points or maybe SSR is too new to have a good formula available to predict it. Many new purchasers will want to use their points at their new home right away, so maybe they are more conservative about releasing rooms for SSR right now.
It's possible that they will release rooms for a member cash discount as it gets closer to the dates.