It's about continuity on the offensive side of the ball in a offense driven, points driven league in which the rules are geared toward the offense. It's easier for an offense to continue on in the same system, with the same primary offensive mind and play caller than it is to start over with a new primary offensive mind/system/playcaller.
If a team's HC is defensive minded, and they are successful and winning because of their offense, their OC/primary offensive mind/play caller is going to be going out on HC interviews. If he gets a HC job, he's likely to take the most effective members of his offensive staff with him to his new team for promotions, they are his guys, not the defensive HC's guys.
And then, the following season, there is the team with the defensive HC, coming off winning season, scrambling to come up with a new offensive staff in an offense driven, points driven league, where the rules are geared toward the offense. Unless that team is coming off a SB win, they are further from winning the SB, not closer to it, because their players are going to be learning a new system, "thinking and not playing," etc, etc.
This has happened twice with the Bears since Ditka. Wannie had the Bears best offense of the modern era in '95 with Ron Turner calling the plays. Turner left after 96 season for the Illinois HC job, in '97 the Bears took a huge step backwards. In John Fox's first year (6-10), Adam Gase coached a moderate uptick in Cutler's performance, and was plucked. The next year they took a huge step backwards, and finished 3-13. Neither the 1995 or 2015 Bears made the playoffs, yet their OC's were promoted elsewhere, and the Bears offenses immediately floundered.