A quick #thread on why:
law.cornell.edu/supct/html/08-…
But the bar for mandamus is _much_ higher—requiring the claimant to show a "clear and indisputable right to relief."
Instead, it used the specter of that concern to conduct ordinary appellate review of the subpoena's compliance with the Federal Rules of Civil Procedure.
lawfareblog.com/judge-wilkinso…
Talk about "judicial activism"...
/end