I'm an idiot and this may be wrong, but does this make any sense? A defensive head coach usually means a coordinator is the architect of the team's offense. And if that team is successful, it probably means they have a good qb and the offense is successful, because offense is just more important these days (I think). Which means that coordinator won't last before being hired to a head coaching job. Which means you need to bring in a new offensive coordinator with his own new offense. The reverse isn't as damaging to consistency, because who cares about defense. Basically, QB is by far the most important position, therefore offense is more important, therefore you want an offensive head coach? Obviously being a good coach is more important, but you don't really know that ahead of time. Like I said, I'm dumb and maybe this is dumb - feel free to let me know.