If an established record was the requirement for the Supreme Court, the founding fathers would have written that condition in.
They did not, leaving the Court open for nominees from all walks of life and not just judges with established records.
Because the power of the Courts have been so badly abused, some of us would like to have substantial evidence that a candidate is not going to compound that problem by engaging in judicial activism.
The Founding Fathers didn't envision penumbras and emanations and foreign precedent and so forth.