Because most NFL head-coaching searches are shotgun weddings conducted by semi-interested billionaires with far more luxurious activities and lucrative businesses to tend to, there is an overwhelmingly large chance that the relationship fails and, a few years later, they’re back in a board room passively listening to a candidate tell them exactly what they want to hear.
When a coach actually delivers on promises, reaches a Super Bowl and makes the playoffs on a semi-regular basis, that should be an inclination to keep that person.
Because most NFL coaches are hired due to their specific expertise on one side of the ball (in Dan Quinn’s case, the defensive side), they fail to create something all-encompassing early in their careers.