Why do judges and Supreme Court justices wear black robes?

Not all judges wear black robes. Judges technically do not have to wear robes at all. The robe and it's color is traditional and customary only.

Some judges now wear dark blue or navy robes. Chief Justice Rhenquist wore black robes with gold stripes, a fashion he took up after playing the role of a judge in a Gilbert and Sullivan performance.

Depending on the court or the calendar, a judge may wear no robe at all. In some states magistrates sit without robes.

It is a tradition that judges wear black robes only in the United States. Judges in Great Britain wear scarlet robes, as anyone who watches PBS would know. There has been controversy in the last few years in England as to whether some of these traditions (including the wigs for both judges and attorneys) should be maintained.

The tradition of wearing black robes dates back to c. 1635, England.