I should really shut up here and leave the explanation to one of our more knowledgeable cricket fans. However, I would surmise that England only need to draw this match, whereas Australia still theoretically have a chance of levelling the series. To keep the Aussies under the cosh, my guess is that England wish to inflict as much pain as is possible on them with the bat at the moment so that a win virtually slips out of sight for the Aussies and so that their batsmen will be under unbearable pressure when they reach the crease. That said, one of our cricketing buffs is likely to come up with a better explanation!