Reasons Why:

Reluctance of collectors to sell at auction because of lower prices and higher buy-ins.
The blockbuster masterpieces are not appearing at sales.
Higher costs passed on to buyer with extremely high (insulting) buyers premium of 25 percent of the hammer. Collectors revolt, less buying at auction, and buying at lower prices.
Sothebys is relying more on the internet, but you cannot bid through them. Broken internet model.
Sale catalogues are getting very thin. They cannot entice and get the material.