The obvious answer is that guys who have options aren't scrambling to take these jobs.
They know O and Chin professionally. They have differing degrees of personal relationship with them.
Why do they not want the jobs O and Chin left? Probably for the same reasons O and Chin wanted out.
Chester shouldn't be lumped in, in this analysis. He was working for a guy even older than Brad, with even less high major experience than Brad. Maybe things are looking up in Blacksburg, but they took over for Buzz, who'd stocked the cupboard and gone to three straight tourneys before he bolted.
Chester has seen a coach succeed, at first, with a previous coach's recruits. Twice.
Champaign is comfortable for him and gets his wife closer to her friends and family. It was an easy choice.