The main reason being Rivera's impact value.
The value of Rivera is much higher with a lead than it is when the score is tied.
Torre MUST cover his arse for tommorrow night. If the Yankees are ahead late, bringing in Rivera is almost locks it up. If he uses him up tonite, he has to get a run to create value in the move. If he uses Rivera up tonight, rendering him unavailable for tommorrow and the Yankees don't score, Torre wastes his ace. That BG, is the condition that kept Mariana out of tonight's game.