When your team is built around defense, your window for success is great but short lived because you can't afford to keep 6 defensive players who all think they are the best defensive player on the team. Also, careers for defensive players seem to be much less than offensive players.
The Texans have arguable the best defensive player in the league and they wouldn't even sniff the playoffs if Andrew Luck doesn't get hurt and miss half the season. They still barely won the division.