Isn't that obvious?
Shula is Rivera's best hope at keeping his job. Not because he is the best OC available, but because he does not have to completely overhaul the current offense.
Hiring an outsider offensive coordinator would mean the entire offensive staff would be gone and a completely new offensive system in place. A new playbook. New terminology for the players to learn. New chemistry to be developed between players and coaches. Often, this can take more than one season to start yielding results.
Rivera does not have more than one season. If the Panthers do not win this year, he is gone. He knows it, everyone knows it.
So put yourself in his shoes. Knowing you can only lose 7 games next season or your ass is fired, would you
1: Hire a completely new offensive staff and implement a new system knowing in all likelyhood it will take a good of time to develop into a good offense if ever at all. or...
2. Promote from within and hope the current offense can be good enough to get you 9 wins. No learning curve, fewer variables.
Lets be clear here. The Shula hire was not made because he was the best candidate. Shula was hired because it presented the best option for Rivera to possibly win 9 games next season. It was a last ditch short term job saving hire. Any considerations past 2013 are an afterthought.
Rivera's goals of winning 9 or more games align with what we want pretty well. The organization wants to win, and the best chance at doing that is this continuity you speak of. The continuity that Shula possessed is exactly the quality that made him the best candidate.
Shurmur sucked. We weren't winning nine with him.
There are other candidates that we could win 9 or more with probably, but none are a safer bet than Shula. In this case, being a "safe bet" won out. It's best for Cam and the whole offense.