There are probably a number of factors into why the sites sign certain players over others. If you go back to 2007's ME, Kravchenko was signed by PS, but the winner Jerry Yang while being listed as a "friend of Full Tilt" never really got super involved with them. (Of course he was a lackluster champion, not playing many events while champ anyway).
Gold wasn't signed by any of the big sites because he had a legal issue when he won the ME and was generally considered to be bad for poker.
Personality can also be a factor into sites signing some people. Location is also a big one for at least PokerStars
. They signed Raymond Rahme (2007 ME 3rd Place) because he was South African.
I think in time you'll probably eventually see these sites begin to drop people. Depends on what their marketing department feels they bring to the company.