The Denver Broncos hadn't played against a top ten-ranked defense all season. The Seattle Seahawks are the top-rated defense in the league this year.
As it was noted earlier today, of the 12 highest-scoring offenses in NFL history, only five of those teams went on to play in the Super Bowl. Only one of them (the 1999 St. Louis Rams) won the whole thing.
Denver has the highest-scoring offense of all time, but are floundering against Seattle's defense, leaving the world to invoke the ultimate sports chestnut: Offense wins games, defense wins championships.