There are certain inter-operability talkgroups that do operate system-wide. The reason all talkgroups don't roam system-wide is because of resources. The sites in the system don't have infinite resources AKA there are only a certain number of channels per site. For example, the Chief of Police Conference is held in Orange Beach every year. If you had the Chief of Hsv PD, Decatur PD, Birmingham PD, Anniston PD, Madison, and Gadsden PD all there and all wanting to listen to their agency's radio traffic then you just overloaded the tower at Orange Beach and now the normal users at that site have to contend with queuing and busies that they normally wouldn't have to worry about.
As an Admin on the system, I can tell you that the sites in Morgan Co are busy enough at times that I don't want or need traffic from an agency six counties away tying up channel resource so one guy can listen to while he drives from wherever to Huntsville. I can also tell you that it would be nice if we could let all of the talkgroups freely roam but aside from the technical reasons stated above there are also (as in all gov't) political factors involved as well. It would be really convenient if ambulances transporting from hospitals in Morgan Co to hospitals in B'ham could call on the radio when they are at their destination and when they are clear and enroute back. Right now, they can't on their normal dispatch channel. Could dispatch monitor one of the statewide talkgroups? Sure, but now it is a training issue for the dispatchers and all of the ambulance personnel. And it is one more thing the dispatcher has to keep turned up. Could the system owners do a better job of facilitating roaming of certain talkgroups and allow more roaming? Sure, and they probably should but first-hand knowledge knows that there are some personality challenges that would have to be overcome and political challenges as well. Then the last thing is money. The owners of the individual systems have put a lot of money into their systems for the purpose of serving the public safety community in their county, not a county halfway across the state. Some system owners would be happy to let anyone roam, within reason, for a fee.
No it isn't perfect, but it's what we have. Since the State could never pull its act together and build a statewide network, individual counties and cities have done what they can to build a system. Oh, and the Alabama First Responder Wireless Commission?? It is a board whose sole purpose is to guide and make recommendations to the governor regarding the First Net Network, Although many of the system owners are members of the board, the board itself ha nothing to do with a Statewide P-25 radio network.
Disclaimer: My posts are my opinion only and do not necessarily represent the views or opinions of my employer.