Completely for financial reasons. He is in his early thirties, and will see it as a last opportunity chance to make a quick buck - well his agent does.
Let him go, our last chance to get some top dollar for him, it's clear he is just a mercenary.
Clarke plays him out of position as it is and that's hardly working, so why keep and unhappy player.
Take the money and run. Also, Harry Redknapp, you are a w*nker.