When hiring a G5 head coach, they're usually younger/less experienced, there's a greater implied belief that they may not have hit their coaching ceiling yet. But if someone still looks like a rising star at the P5 level, they're in the realm of being coveted by the richest college programs (i.e. VT can't afford them) or the NFL. That just leaves coaches who have probably already peaked (think like Tom O'Brien, who was never really bad at BC or NCST, but never very good either), or a fired coach from a program with higher/more insane expectations than VT (people like Mullen, Herman, etc.)