I bet it is the internet. The last few San Francisco conventions I attended, it was cheaper to book through the internet at Embassy Suites right near the Cow Palace (close enough to go to for lunch) than to travel 20-30 minutes to use a hotel on "the list" for a run down 2 star room and fight traffic on Friday.
I checked some of the hotels on the Societies list and they had a travel agent's booking commission of 10%. I would not be surprized if "booking Commissions" had more to do with the list than any other factor for WTBT$.