One reason might be to prevent injury by playing a full 5th quarter. Another 15 minutes of football after 60 minutes of football means a lot of fatigued guys running around. Likelihood of injury probably is increased. Sudden death ends the game once someone scores, so typically the game will end without another full 15 minutes of playing. Source Link :- http://b4tea.com/sports/other/nfl-overtime-rules-details