A couple of reasons... one is frequency allocation has to be careful as a result of skip. The range is why people choose it over VHF, especially when they are unable to repeat. I know in CT the state uses a low band repeated system for parks, for example, but a department or organization that is wide area and can't exactly afford to start knocking on peoples doors and buying property for a repeater site - so what is the point of repeating a system if the receiver has to be at the dispatch office? That is why low band is often picked in lieu of one of these, so they never usually end up repeated.
I think the reasons for AMR has to do with the way it operates. Low band is south part of the state, and VHF-Hi for north part, but I think they keep it un repeated because there are different dispatch centers and at the time, they chose what was easiest without having to buy repeater sites because it is much easier for a government agency to buy a repeater site as apposed to private business. The way AMR makes money I am suprised they have started to work on some sort of VHF/UHF trunking system that would allow them to have networked capabilities and run talk-groups for the different areas.
