Look around the league at the defensive head coaches. If you take out the 3 interim coaches (Det, Atl, Hou), you only have 9 of 29 coaches who are defensive guys. Of those 9: Vrabel, Tomlin, Carroll, McDermott, and probably Rivera and Flores would make 6 of 9 of them leading playoff teams. The ones not making the playoffs are Fangio (devastated by injuries), Zimmer (who still could make a run for the playoffs) and the greatest coach ever, Belichick.
If you go for a guy simply for the purpose of developing a QB, you're much more likely to end up with a Matt Nagy type rather than a Kyle Shanahan type. Thats not to say you don't go for an offensive head coach. You go for the coach you think is best able to carry out the organization's vision (assuming the Bears get one) and don't limit it to "oh we need a guy to groom us a franchise QB".