re: How?

It is because a DC will have a much more difficult time transitioning to the HC position than an OC. It takes years to perfect a defensive scheme and in college you have kids start 2-3 years and move on. You're constantly starting over so unless you're Bama, it's extremely hard to perfect your scheme. So DC's are more closed off, stubborn and resistant because they have spent their careers trying to do the impossible.

An OC can have an impact almost right away, especially with the spread variances. An OC learning curve is so much easier, especially if you recruit a good QB. There is no single recruit on defense that can have the impact of a good QB on offense. A good OC is flexible, and uses his offense to cover up deficincies in personnel, and exploit strengths. OC's attack defenses, DC's learn to respond to attacks.

Successful first time HC's overwhelmingly come from the OCs. DC's overwhelmingly struggle in their first HC job.