There are a number of good reasons why public agencies hesitate to upgrade. First, there are so many of them (agencies and radio systems) around. Yes, the systems are antiquated, and easy to listen or interject into, but given the many millions that are deployed out there, you will incur a huge cost for replacing the existing system. This is complicated by the fact that you often have to use a different frequency band altogether, which could require the use of new base stations, cabling, antennas, etc. If the frequencies are too far apart, you also have to replan your network to account for the gains from technology (channel planning, capacity considerations, network planning, other services you may want to add, etc.) and the losses from possibly moving to a higher frequency.
Second, you cannot compromise public safety during the transition period. This not only applies to the agency that is doing the upgrading, but also to all the others who rely on that agency to provide services. Finally, you have to contend with the chaos during the transition period, when you either have two, probably incompatible, systems in operation. There is also the problem of finding available spectrum and relocating existing users from that band you want to move into (This happened when cellular services were introduced in the US in early 90's and the existing terrestrial microwave links had to be relocated from the 2GHz band to the 6 GHz band).
Yes, many public systems live in the stone age, but it is not because they like living there. There are often very few paths available to modernize cleanly and efficiently without spending a lot of time and money and not risking public safety.