Because it was his plan all along. Not me saying so, Mr Booth post fight in a video interview on the Scene of Boxing. In it he clearly states they'd noticed that the more punches you threw at Valuev the more that you got hit, so the plan all along was to win throwing as few punches as possible. That was post fight, no broken hand excuse at all. A swollen hand he may have had, but the plan was simply as stated.