It hasn't always be like this, but these last years, the last stage is not decisive for the victory. The general rankings stay basically the same as the day before.
While officially racing in Paris, riders take pictures, drink champagne so this stage is more like a celebration and the stage victory is left to the final sprinters.
Is there a reason for this? Are there some implied rules to let the rankins stable on this day and not try to attack the yellow jersey?