Contrary to popular opinion, the Vegas line on a football game is NOT a prediction on who is going to win.
It is set so that exactly half of the bets are on one team and half on the other. That way the sportsbooks cannot lose (because of the 10% "vig").
The line is a reflection of the perception (or perhaps "misconceptions"), of the betting public on who is going to win. Not a prediction, although often it works out that way.
Many people have the mistaken idea that because the AFC was a stronger league overall this season, that any AFC team will beat the best NFC team.
The Seahawks, while in a weaker overall league, were the second best team by record in the NFL during the regular season. They also had a much more difficult schedule than the top team (who are now dead) and enter the Super Bowl on a playoff roll with nothing but dominating victories.
The best team in the NFC beats the sixth best in the AFC. No question.