Why is there an AFC and an NFC? Why isn't there just all the teams under just one NFL?

Because for instance the Dallas Coyboys can never play the New York Giants in the Super bowl because they are in the same conference. And also teams from different conferences can never play each other unless its in the Super Bowl. Why cant there just be one conference, and they all have a shot at the superbowl, and that way, each team might have a chance to play any other team during the season.