It's an EDACS system with analog and ProVoice modulation. Darn near everything is ProVoice which can not be decoded by any scanner. A few talkgroups, like the dog catcher and some other city services, as well as Fire Dispatch (the dispatches only - the rest of the traffic is ProVoice) are analog and can be heard on a scanner.
Any EDACS capable scanner can trunk-track the system (i.e. know which talkgroups are active), and any EDACS capable scanner can listen to the analog talkgroups, of which there a few, but NO scanner can listen to the ProVoice talkgroups, of which there are many.
It's not the technology that keeps manufacturers from selling ProVoice scanners... it's the patents. I'm sure that Uniden and GRE have scanner designs sitting in their labs right now that will decode ProVoice, but unless and until MA/COM decides to reasonably license the technology and until the scanner manufacturers (and by extension, buyers) are willing to pay for it, there will be no such device.