no one else was getting any swing
Siddle got a few to swing a lot early on, maybe not later. I didn't really watch Hilfenhaus closely but I assume he swung more than one. Harris definitely swung it at the end, away from right handers; it was how he dismissed Bell in the 62nd over. That was surely just as strange as Johnson swinging a newish ball.
There was also swing there for England on day 1, but they did start to bowl quite a lot of short balls after the first hour. Maybe they got impatient and missed out. Part of the brilliance of Johnson's spell was that he landed those balls right on the money. They couldn't be left or played at and missed, nor top edged for six. When so many swing bowlers drop short of a good length, it was surprising that such a canny performance came from this player who is known as much for a lack of swing as he is for what he can do when it does swing.
While much is made of the pace and bounce, sometimes the swing is one of the underrated characteristics of the WACA Ground. It was after all what won the game for India in 2008. The Doctor is a good steady breeze to bowl with and the dimensions of the ground allow it to flow through. They've also recently planted a more delicate grass on the pitch square, with a view to aiding pace bowling. It seems to have done just that, not only helping the ball to skid through, but perhaps also allowing it to hold its shape and shine for much longer than on most Australian grounds.