Disney is currently selling Riviera Resort which they built. Just a guesstimate, but lets say it cost 35$ per build. That is, they are selling something for 180$ they paid 35$ for. Why buy something for 120 to sell at 180?
Also, its just business. Why would they want to tie up cash in excessive inventory? Businesses want "just in time inventory", they want the last of what is being sold being taken just as they are getting their new supply in. It lower's their carrying cost.
Think of it this way - a store selling widgets has 10 of them - their ideal goal is to be getting a new shipment of these widgets in just as a customer is purchasing the very last one. They dont want 10 more right away when they already have 10 - thats money they can not do something else with.
Its the same for Disney. They push the new/current resort because it is their highest profit margin. If they pushed all resorts equally, the points would be spread out more equally across all the resorts. They would then have to 'buy back' (ROFR) more points on the resale market to meet their sales. They would be selling less of those Riviera points that the maybe paid 35$ each for, and selling more Beach Club points that they just bought back at 135$. Which would you rather do?