Peyton Manning has had a run of statistical success during Caldwell's time with the Colts ('02-06) that has rarely been seen since the invention of the forward pass. While it could be argued that Manning pretty much coaches himself (especially in preparation), Caldwell most certainly has his ear and helps Manning on the sidelines between drives.
Caldwell has plenty of big game experience, since the Colts have been to the playoffs every year he's been with the team.
He has head coaching experience, having coached Wake Forest from 1993-2000 (before joining Tony Dungy in his last season in Tampa).
He has worked with men that are highly successful and well respected (Dungy, the venerable Joe Paterno, and Manning).
Chances are, Edgerrin James recommended him to management. That's the only link I could find. If Edge likes him and respects him, the other players on the team will like and respect him, since they like and respect James.
He has no other interviews scheduled. I had to Google him, which means that I hadn't heard of him at all before the Cardinals decided to interview him. I watch a lot of football and follow the off season moves of every team. If he slipped off my radar (and the radars of the other teams in the league), how good can he really be?
The Colts are still alive in the playoffs. The longer they survive, the longer the Cardinals go without naming a head coach.
While he's currently an Assistant Head Coach (as far as I'm concerned, a "paper title" to keep assistants from moving to other teams to become coordinators), he's never been more than a quarterbacks coach in the NFL. He has head coaching experience, but he hasn't had a great deal of experience with an entire unit, nonetheless an entire team. Plus which, given the fact that Manning has started every game for the Colts since the beginning of his career, how much time do you think Caldwell actually spends with Jim Sorgi?
It could just be that Edge wanted to do one of his favorite coaches from his former team a favor. Caldwell does have a pretty impressive background, some head coaching experience, and he does coach a marquee player at a marquee position. Edge might have wondered why no one was interviewing Caldwell and recommend him to management.
The Cardinals had scheduled plenty of interviews; what was one more on the pile? In the future, Caldwell will probably garner interest, and deservedly so, with other teams. Plus which, it seems as though the Cardinals are an open book.
They've been burned by big name coaches and up-and-coming assistants. If Caldwell blew them away in the interview, I don't see why he wouldn't get the job.
But, I also don't see how he could've gotten it. The Cardinals interviewed hotter candidates and people with more experience. They'll settle on one of those and watch as Caldwell becomes a "hot name" next off season.