I have done some fast google 'researching' about some top vendors who is not longer with us. It seems that they have not so much quit as moved on to greener pastures. AS Shanim is working on 4 different game projects and Sanctum Art is also in the game business and even film. Xurge has his own successful site.

What I could gather by reading between the lines is that the top artists is bothered not so much by oddities and policies of the vendor as tiring of a few vocal and unreasonable users.