Why is the current court made up entirely of former federal appeals court judges?

Answer 1

Most of the current justices were placed on the US Courts of Appeals Circuit Courts specifically because they were considered potential future Supreme Court justices. This not only gives the justices appellate court experience, but also helps build a record of jurisprudence that allows both the President and Senate to learn where they stand on key issues, how they're likely to vote, and what kind of reasoning they use to support their decisions.