The way I heard it, the Providence Journal (and passed to Belo and now Gannett) had the rights to the KING call letters and after the TV and radio stations split, they agreed to share the KING call letters. When the Bullitts donated KING-FM and sold KING-AM, PJ wanted KING-FM to change their call letters and backed off when there was enough of an uproar from the arts community. KING-AM however didn't share that luck.
And to this day, there is still no KING-AM (or just KING) anywhere else because PJ and subsequent KING-TV owner Belo would not allow another broadcaster outside of KING-FM to have them, even though I'm sure they've been requested.
Looks like Gannett is a little more flexible....