Castle is so FAR left, if he had won we would already have lost. Now we still have a shot. This tremendous primary victory will greatly improve O'Donnell's momentum and vastly increase her chance for victory in the general election!
And it's a race conservatives across the nation will be focused upon and support financially and in a myriad of other ways.
And in picking Castle, would also have sent a message that Castle's position was agreeable. A sort of "compound losing."