I would monitor their old TRS and see if ANYONE is using it. Might just be down. Maybe they had issues with it and depending on cost they might be using the 8Tacs since they're on repeaters, I know that all the radios fire, ems, police or whatever have the 7Tacs and 8Tacs already in them. Maybe since they are close to going to new system they didn't want to renew license for frequencies on old TRS. Would be extremely easy to just say screw the old system and start using new radios since everything's already there. Who knows. Departments do strange things and sometimes for stranger reasons.
Might be the new radios. I know for a fact that when we got our radios in St. Louis county we were told to use them immediately. Even though our older radios worked fine for our needs, we abandoned them literally as soon as we got the new ones. Reason: some buttons and functions are different, as well as tons of added zones and interops. They wanted us to use the 7Tacs on scenes but our dispatch does not have capability to monitor that and they still haven't figured out the emergency button situation yet. They want everyone on same page.
If you ask me, St. Peters getting new radios but staying on an old TRS would be the exact opposite of maintaining interop with these new radios.