topr8 - 8:41 am on May 30, 2012 (gmt 0)
for the obvious reason that they got in early and are the huge.
as a buyer - why bother to look anywhere else as the most stuff is on ebay.
as a seller: why list on small sites with few visitors.
however i notice, that many ebay sellers in th ecollectible fields now list their items in multiple places, although usually not other auction sites, but there are many fixed price specialist sites where you can list your items within a particular niche.