They didn't have enough to convict under the law as it exists:
The case was reviewed by a deputy prosecutor and ICAC (Internet Crimes Against Children) investigators, and it was determined that the images did not meet filing standards for exploitation crimes against children, Dan Donohoe, a spokesman for Prosecutor Dan Satterberg, wrote in an email Tuesday about the 2013 investigation. As a result, we would not be able to prove a case beyond a reasonable doubt and charges were not filed.
Source:
http://www.seattletimes.com/seattle...loitation-charges-against-3-elderly-brothers/
Of course, bystanders like us can argue all day long about whether they took the right action, but it sounds like they didn't have the evidence they needed back in 2013. And the admission of guilt applied to crimes where the statute of limitations had expired.
This is an absolutely horrifying case, and I wouldn't be at all surprised if we hear worse as it is investigated. The number of victims could be huge.
And I can't imagine the horror of being the relative who found the obscene materials. Three lifetimes of unimpeded crime - when I think of the victims, it's just heartbreaking.