The first use of radio by a fire department occurred in Boston, MA. Several of the Chiefs vehicles were equipped with maritime mobile radios to communicate with the cities fire boat. Dispatch also being equipped with a maritime mobile shore station everyone involved quickly fell into using it between the dispatch office and the chiefs cars. The FCC quickly stepped in and ordered the city to stop but the chiefs were hooked so the city got one of the earliest two way public safety licenses.
Police dispatching was originally one way and used modified AM car radio receivers to announce calls to the prowl cars. Being only slightly out of band for am broadcast it was very cost effective to order the cars with extended range AM receivers to make the system work.
The conversion to digital is driven by the supposed cost savings of using a single system to support all local government radio use. The problem is that trunked radio is the predecessor to cellular telephone service and all of the problems attendant to that technology are in trunked radio. The biggest of these is "dropped" connections. When a digital signal is on the edge of effective it is either there or not. When the signal is marginal it drops in and out of readable. To public safety workers that is unacceptable but the bean counters don't care. They blame all problems on the users resistance to change. They're not the ones crawling down long, snotty hallways at Oh-dark-thirty looking for other peoples' relatives in zero visibility, so they don't understand why we complain about not being able to talk to the people supporting that effort for minutes at a time.
Also the savings are illusory. Motorola's best portables cost only $600 ten years ago when purchased in quantity. The portables we now carry cost $5000 dollars a copy. So the cost of portable radios for a crew of three has gone from $1800 to $15000 in ten years. At these prices $150 dollar hammers seem cheap. All of the early adopters are being robbed because once they select a vendor they are locked into a proprietary scheme that there is no alternative for. The smarter and poorer agencies are waiting for the standards process to shake out. Once standards based APCO 25, or whatever standard replaces it, equipment becomes available then it will be financially feasible to adopt digital signaling with portable radios that only have to reach the vehicle that brought you to the incident. The more powerful outdoor mobile radios would then do the heavy lifting of reaching the tower site.
As for talking to the cop shop directly there is a organizational culture problem to overcome. The two organizations do not speak the same operational languages nor do they share the same operational priorities. We have so narrow a focus on alleviating suffering that we sometimes destroy the evidence that the police need to prevent a recurrence. They are sometimes so focussed on identifying the perpetrator that they neglect the very real suffering of the victim. They won't even call us if the victim isn't bleeding for fear we will follow our training and put the victim in touch with a victim support worker thus depriving them of control of the witness. One of the options victim support will expose them to is not pursuing the conviction of the perpetrator for the sake of their own mental health. That really ruins the detective's day.