The Seattle system and the King County/Valley Com systems are different.
At least they were when I worked for King County. (I'm retired from there)
Both systems shared some "resources" tho.
Some special assignment people have their radios programmed to be "native" on either system.
Probably both systems SHOULD have been the same system.
Maybe that is the direction they will go.
Last time I worked details in the tunnel, my radio worked fine on KC/VC, but I never spent much time down there.
And, some of the infrastructure is getting old now.
As one of the above posted, likely they are looking for funding.
Unless there has been major changes, I think a lot of that system is maybe a dozen years old.
And, after working for KC for nearly 30 years, I know they OFTEN have trouble funding things.