In playing conditions, the human heart and body are more likely to give out during cases of extreme heat than extreme cold. Maybe you might have a point in -57 degree weather, but that's a rare condition. Even then, if the NFL begins postponing games (especially playoff games) because of temperature, then where will that end? At what point does the NFL say that the tempurature is just warm enough to play a game? Technically, freezing temperatures begin at 32 degrees. If we're worried about human health in cold weather games, and looking to end games that are played in that cold of weather, then many of the best games in NFL playoff history would be cancelled. Immediately, you would have to cancel some of the great Patriots games such as the Snowl Bowl game against the Raiders and the AFC Championship against the Colts (two games which we won at least in part due to the weather). All time great games such as the Ice Bowl would have had to been cancelled or postponed. Where would it end? At what point would we say "yeah, they could play in those conditions, but not those". Any northern NFL team entering the playoffs would either have to construct a dome, postpone their games (good luck finding acceptable weather conditions down the line in December and January), or move their games away from their home stadium to a warmer southern climate.
In all, at what point do we stop ****ifying the game? To my knowledge, there hasn't been a slew of deaths in the past related to the weather. Why would we try to control it now? The game would immediately lose some of it's playoff lore. And, after all, who DOESN'T like watching a snow game?