Ostensibly, the same reasons why the AD might believe Red or Gerry are 'qualified' to be the next head coach would be the same as those when Hopkins was designated HCiW. The same as for Scheyer at Duke. Boeheim himself. The same for any assistant promoted in any program in any sport throughout history. The same for anyone in any job in any industry who gets a promotion. The person has demonstrated "acumen" and intelligence and passion and competency at important skills. That's not "proof" they'll succeed in the next chair, but neither is success at a small program, leading to being labeled 'the next big thing.' It's probably more comfortable to believe that HC success on any level is proof, but that's as context-driven as failure. And, with bringing in an outsider, you're relying on a resumé, not a relationship and personal experience.
That all said, i'm not campaigning for the internals. Actually, having seen a few hours of teams in this tournament, i'm more fully on the side of wanting completely new blood and a complete overhaul. It's not that Red or GMac can't correct the course, but i fear they're both too ensconced in an ethos that is old hat, and i can't trust that they'd be inclined, inspired, or capable of breaking out of the JAB mold.