This is all pretty stupid.
For many dozens of years before the tunnel closed, Metro operated on a UHF (450 MHz) conventional analog system. Since ST didn't exist, there was no need for interoperability. The system had expansive coverage throughout King County and beyond, and even had dedicated repeaters in the tunnel.
Since its inception, Link has used an 800 MHz trunked system (PSERN). It's the same system used by every police officer, firefighter, and loads of others in King County.
When joint operations started in the DSTT, Metro had to install a second radio in every bus that went in to the tunnel. Apparently having train operators and bus operators able to talk to each other prevents crashes (or something like that; probably some sort of rule set out by the FRA).
In 2011, Metro replaced their old UHF system with a 700 MHz trunked system. Despite only being 300 MHz apart, the two were nothing like each other and wouldn't be able to use any of the same equipment.
The 700 and 800 systems aren't terribly different from each other. From a technical perspective, there is no reason why Metro couldn't have been put on PSERN. However, voice transmissions on transit systems tend to be frequent, long, and drawn out. PSERN didn't have the capacity for that, so Metro had to build their own.
Both systems use Motorola radios. Motorola sells radios that will do 700 and 800 in the same radio. I'm 99% positive KCSO Transit PD carries these so they can talk to the PD dispatchers and the Metro coordinators without having to have two radios on their belt. The same could have been done in every tunnel bus. But for reasons of timing (and maybe some systems integration issues) it wasn't. So now every bus that goes in the tunnel bus has to get an extra radio installed, and at the end of joint ops there's going to be a ton of surplus 800 radios.