Tradition, but that hardly answers your question.  If you search around the Internet, you’ll find several websites claiming that the black Robe Tradition Started in 1694 with the death of Britain’s Queen Mary II.  The story is that Britain’s judges donned Black Robes in mourning for the queen, the attire propagated through the British colonies, and that is why judicial black robes remain in most former colonies of Britain.  There may be something to this story, but it appears a little too neat to me.

There are images predating 1694 of legal officials wearing simple black:

Clearly black robes were “in the mix” so to speak before Queen Mary II died.  It is possible, however, that the official mourning period coupled with Britain’s rapidly increasing colonization gave impetus to what was already an existing theme.

After the American Revolution, Thomas Jefferson and others favored eliminating judicial robes, because their association with the British aristocracy offended egalitarian sensibilities. Jefferson thought judges should wear suits like other citizens, which is funny when you realize that robes probably evolved from tunics, which were originally the dress of common people if you go back far enough.   John Adams wanted to keep the distinction of special judicial attire, including the striking red robes and white wigs favored in the British courts.  Adams was a lawyer, by the way.  British judges were wearing red in the late 18^th century, nearly a century after the death of Queen Mary II, which also calls into question the degree to which the black robe tradition started with her death.  The founders’ compromise was to forgo the white wigs and keep the robes, although it isn’t clear that black was necessarily the preferred color at this time (see John Jay portrait).  Further decisions regarding judicial dress were reserved to the states.   By the 19^th century nearly all US judges wore simple black robes, as they still do today.  Some people consider black to represent neutrality.