A lot of times great coordinators can not transition successfully to a head coaching job. I just don't think some of them have what it takes. Their best destiny is to be a coordinator. I've thought, and said this many times over the past couple of decades. You see it rather frequently. A couple of names come to mind; Norv Turner, Jim Schwartz, Dan Quinn, Wade Phillips. A couple of those had marginal success as head coaches, but they're still better at the coordinator job.