The defense can't advance a fumble that is blown dead. The reason it wasn't looked at was because it was 4th and all a review would have done was just give LSU the ball at the spot of the fumble. Since it was a turnover on downs there was no point in reviewing it. Basically the refs blew the play dead so the logic behind it is because there was a whistle, the ball can't be advanced if replay shows it was in fact a fumble.
