From what I recall, the murders stopped abruptly and no similar ones have ocurred. There also is the belief shared by John Douglas, among others, that the Green River homicides were not all committed by the same person - that several serial killers may have been operating in the area at the same time, independantly of each other. FWIW, I think that’s probably the most likely answer. There were enough differences among victims, injuries, etc that it is more than likely that they were not all killed by the same person.
AFA having only one officer on the case, well, it’s been 15 years since the last murder. My guess is that the officer in charge is doing a cold case investigation. With no activity in so long, the department probably can’t justify a huge investigation - heck, they did that, with no luck before.