There are many reasons. Perhaps the primary reason is no franchise management of any kind.

Obviously he's a terrific field manager. Being a Hall of Famer (soon) does not translate to being a great executive. It would take him 2-3 years to figure things out and, in the meantime, the Colts will continue to be mediocre at best. I could go on, but perhaps the biggest reason is that he will likely turn down the opportunity. I'll let you fill in the blanks but look what is happening to Andrew Luck--great player, questionable GM so far.