On top of being a tougher player to replace and a cheaper player on the market, Callahan is likely a better scheme fit for Chuck Pagano. The buzzwords around Pagano since his press conference were aggressiveness, attack, and swagger. He wants to blitz more and has been noted for blitzing his defensive backs.
According to Pro Football Focus, Callahan is coming off of an elite pass rushing season. He was charted for 13 combined sacks, hits and hurries. Adrian Amos had thee combined. If Chuck Pagano were to pay for a high-end safety, he would want a player like Landon Collins before Amos. However, Callahan is the type of cornerback Pagano would be attracted to.
With that in mind, the Chicago Bears are much more likely to keep Bryce Callahan than Adrian Amos. He fits the ideology of what the new defensive coordinator wants to do. He also does not have a young player to step in for him on the roster. Add in that he is the cheaper option and the Bears should be looking to Callahan and then seeing if they squeeze Amos in after.