Because a lot of the time, the ball doesn't swing out of the hand. When Watson was swinging the ball, even at his pace, it was swinging fairly late. In those cases, if the ball isn't holding its seam position after pitching, it just won't be able to swing late. The trick is getting the ball to hold that seam position after pitching, the 'skiddiness'.Bollinger is meant to be a swing bowler but for whatever reason he can't replicate it. Siddle has swung it before and if you look at his seam position there is no reason why he shouldn't get it.
Because a lot of the time, the ball doesn't swing out of the hand. When Watson was swinging the ball, even at his pace, it was swinging fairly late. In those cases, if the ball isn't holding its seam position after pitching, it just won't be able to swing late. The trick is getting the ball to hold that seam position after pitching, the 'skiddiness'.