Apparently because one or more of the the Wisconsin Supreme Court justices were corrupt and failed to recuse themselves:
[quote]In 2015, Justice Prosser refused to recuse himself from a case in which
the state supreme court sat in judgment over the John Doe investigation,
despite the fact that the investigation focused on precisely the same
network of lobbying groups and donors that had helped him hang onto his
seat. The judge joined a majority of four conservative justices who
voted to terminate the investigation and destroy all the documents now
leaked to the Guardian.[/quote] –The Guardian
