Anyone who is in a position to hire people knows that when possible you hire knowns over unknowns.
Other coaches may have done well at their universities, but that doesn't mean that they can or will do well here. Spurrier had Florida set up as a perennial contender, South Carolina, not so much -- not terrible, but not NC material either. Same with Lou Holtz and Notre Dame, Mike Price (now at UTEP), Steve Kragthorpe did wonders at Tulsa and hasn't impressed anyone at Louisville (though if the kids haven't lost faith in him I think that one will turn around next year), etc. Knowing that, everything becomes and educated guess. We could always look at coaches that have done well at multiple schools, but we don't want to be a step on the coaching carousel, so that won't work. What's left? Hire a known entity that understands the toughness required of Nebraska football teams, and has demonstrated the ability to teach that toughness to others while at Nebraska, and has proven that he can turn a (granted, small time) football program around by instilling that toughness.
I don't envy Solich's position when he took over. It's always tough to come out of the ranks into an upper management position, because the members of the ranks look at you as "just one of the guys", and the appropriate respect for the authority of your position doesn't exist and is difficult to obtain without alienating the people that you need to work for you. I don't believe that either Pelini or Gill would have done well if they were put in Solich's shoes. Callahan's tenure at NU will ensure that the next coach, Nebraska ties or not, won't be put in that same situation.
I have no problems with Gill, I'm not really excited about Pelini, but I would take a "wait and see" attitude rather than taking up arms against him.