When the NFL is in an offensive evolutionary period like we're currently seeing, there is a feverish push to find the disciples of the most potent systems and hire them for any and every vacancy. So it’s not a surprise that over the past few years, we have seen the branches of the Andy Reid, Sean McVay and Kyle Shanahan coaching trees get shaken quite vigorously.
Because of the laser focus on scoring points, for some reason there is less of a focus from owners and search firms assisting them on stopping the points from being scored. Along those same lines, there is not a stable of defensive-minded head coaches who develop similarly robust coaching trees outside of Bill Belichick (now that the Pete Carroll tree seems to have withered a bit).