Isn't that a good enough reason? Mel Gibson is poison in Hollywood and with a lot of the viewing public right now. His last film was a dud. After 'Inception' DeCaprio is flying high and can have any role he wants, at any price he chooses to name. Signing on to a Gibson film certainly could jeopardize that.
That’s my opinion, too. Not every choice isa political one, even in Hollywood. I bet most choices are decided on fiscal grounds.