One word: repeatability. Bad Boy Buddy's action lacks simplicity - too many things have to go right for him to consistently kick straight. Brad Green was prone to have bad days, too, because he relied on a curve in flight. The straighter you run in, kick through the ball and keep your head down, the more chance there is you will kick straight regularly, and the less chance there is you will confuse you muscle memory.
Mitchell Johnson has the same problem with his bowling.