There are several things going on here. First, there are only a few jobs that really qualify someone to serve as a district judge: being a federal prosecutor, a lawyer licensed to practice in a federal court or an academic who has spent years and years studying case law and procedures.
Below that are state court judges who have experience with rules of evidence and defense lawyers.
Top-notch lawyers don't want to be judges because it pays less.
Peterson appears to be a bright guy, from his record, but federal judges have to know both criminal and civil procedure. He may be able to learn on the job, but at what cost? Really, if a lawyer doesn't know what a motion in limine means, some alarm bells should go off.