Fifteen in a row is alot of games to win... Besides, everyone knows that the Pats only won their last SB appearance because the country was in a patriotic mood and for the good of the country, the NFL saw to it that the Patriots would win.
Seriesly, it doesn't matter what you did before you got to the Super Bowl, just ask the 2001 Rams what they were thinking when they saw they were going to play a young inexperienced QB in Tom Brady...