I have no insider knowledge of how any agency plans for interoperability as I'm not involved with any agency. However, my theory is that most agencies just don't see the value in planning for events that will require interoperability on a large, or even medium scale. There are lots of other things to worry about these days - staffing, funding, liability, etc. And for 99% of days, there's just no significant need for agencies to talk directly to each other. Perhaps it's because leaders of these agencies don't really understand the technology and how the technological barriers to interoperability can be easily removed. The long-standing habit of depending on dispatchers to coordinate between agencies off the radio is the default mindset. Training seems to be the major barrier now. Many members of smaller agencies likely don't know what the other channels are on their radio's channel knob. Many probably don't know how to change zones. Radios may not have displays, or only allow 6 characters for channel labels. Training and re-training for scenarios that require making those changes on rare occasions is very difficult.
Regarding Charleston, all county agencies have been on a unified radio system for decades. The technology to support interoperability has been in place for a long time, and the mindset is probably different. However, I wonder how often agencies coordinate radio traffic with each other. Law Enforcement is all encrypted now (likely with a common encryption key so they can all talk to each other), but I wonder how often the Sheriff's office communicates directly with municipal PDs. Or with SCHP. Given that the county portion of the radio system there is somewhat segregated from the state portion, I would guess there's not much direct interop with SCHP. There's probably more coordination among fire departments with mutual aid agreements, and they have all of the common incident channels designated specifically for that purpose. How often do FDs and EMS communicate? How often do Charleston County agencies communicate with Berkeley Co, or Dorchester Co, or Colleton Co? Maybe they don't really need to.
It's probably not fair to compare Charleston to somewhere like Pickens or Oconee Counties. There may be more to it than that, but that's my hunch.
