Why does the AFC play the NFC in the Superbowl?

Should'nt the best 2 teams play in the Bowl? Nothing against the Cards but there were teams with a better record than them that did not make the playoffs. Wouldn't it be better if the Steelers played the Pats or the Colts? Or the Cowboys play the Giants? Out of all fairness, sometimes one league is better than the other. All i'm saying is sometimes the superbowl does not always represent the 2 best teams.