Gemara Shevuos 31a says that if two people come to Bais Din one dressed in rags and the other dressed in expensive clothing, we make them dress similarly. Sefer Hachinuch Mitzva 235 indicates that this is not done in his times. When did this stop and why?
Choshen Mishpat - Shach 17:2 indicates in the name of the Raavan that it fell by the wayside since today no one wears a Uztila of 100 Mana.
The Rambam (Sanhedrin 21:5, noted by Rabbi Chayim Dov Chavel in his edition of the Sefer HaChinuch — mitzvah 217 in his edition based on the first printing in Venice) says that the reason why is because we no longer have the ability to do the judgment perfectly.